f2fs: fix to delete old dirent in converted inline directory in ->rename

When doing test with fstests/generic/068 in inline_dentry enabled f2fs,
following oops dmesg will be reported:

 ------------[ cut here ]------------
 WARNING: CPU: 5 PID: 11841 at fs/inode.c:273 drop_nlink+0x49/0x50()
 Modules linked in: f2fs(O) ip6table_filter ip6_tables ebtable_nat ebtables nf_conntrack_ipv4 nf_defrag_ipv4 xt_state
 CPU: 5 PID: 11841 Comm: fsstress Tainted: G           O    4.5.0-rc1 #45
 Hardware name: Hewlett-Packard HP Z220 CMT Workstation/1790, BIOS K51 v01.61 05/16/2013
  0000000000000111 ffff88009cdf7ae8 ffffffff813e5944 0000000000002e41
  0000000000000000 0000000000000111 0000000000000000 ffff88009cdf7b28
  ffffffff8106a587 ffff88009cdf7b58 ffff8804078fe180 ffff880374a64e00
 Call Trace:
  [<ffffffff813e5944>] dump_stack+0x48/0x64
  [<ffffffff8106a587>] warn_slowpath_common+0x97/0xe0
  [<ffffffff8106a5ea>] warn_slowpath_null+0x1a/0x20
  [<ffffffff81231039>] drop_nlink+0x49/0x50
  [<ffffffffa07b95b4>] f2fs_rename2+0xe04/0x10c0 [f2fs]
  [<ffffffff81231ff1>] ? lock_two_nondirectories+0x81/0x90
  [<ffffffff813f454d>] ? lockref_get+0x1d/0x30
  [<ffffffff81220f70>] vfs_rename+0x2e0/0x640
  [<ffffffff8121f9db>] ? lookup_dcache+0x3b/0xd0
  [<ffffffff810b8e41>] ? update_fast_ctr+0x21/0x40
  [<ffffffff8134ff12>] ? security_path_rename+0xa2/0xd0
  [<ffffffff81224af6>] SYSC_renameat2+0x4b6/0x540
  [<ffffffff810ba8ed>] ? trace_hardirqs_off+0xd/0x10
  [<ffffffff810022ba>] ? exit_to_usermode_loop+0x7a/0xd0
  [<ffffffff817e0ade>] ? int_ret_from_sys_call+0x52/0x9f
  [<ffffffff810bdc90>] ? trace_hardirqs_on_caller+0x100/0x1c0
  [<ffffffff81224b8e>] SyS_renameat2+0xe/0x10
  [<ffffffff8121f08e>] SyS_rename+0x1e/0x20
  [<ffffffff817e0957>] entry_SYSCALL_64_fastpath+0x12/0x6f
 ---[ end trace 2b31e17995404e42 ]---

This is because: in the same inline directory, when we renaming one file
from source name to target name which is not existed, once space of inline
dentry is not enough, inline conversion will be triggered, after that all
data in inline dentry will be moved to normal dentry page.

After attaching the new entry in coverted dentry page, still we try to
remove old entry in original inline dentry, since old entry has been
moved, so it obviously doesn't make any effect, result in remaining old
entry in converted dentry page.

Now, we have two valid dentries pointed to the same inode which has nlink
value of 1, deleting them both, above warning appears.

This issue can be reproduced easily as below steps:
1. mount f2fs with inline_dentry option
2. mkdir dir
3. touch 180 files named [001-180] in dir
4. rename dir/180 dir/181
5. rm dir/180 dir/181

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>