btrfs: fix warning while merging two adjacent extents

When we have two adjacent extents in relink_extent_backref,
we try to merge them. When we use btrfs_search_slot to locate the
slot for the current extent, we shouldn't set "ins_len = 1",
because we will merge it into the previous extent rather than
insert a new item. Otherwise, we may happen to create a new leaf
in btrfs_search_slot and path->slot[0] will be 0. Then we try to
fetch the previous item using "path->slots[0]--", and it will cause
a warning as follows:

	[  145.713385] WARNING: CPU: 3 PID: 1796 at fs/btrfs/extent_io.c:5043 map_private_extent_buffer+0xd4/0xe0
	[  145.713387] btrfs bad mapping eb start 53370886 len 4096, wanted 167772306 8
	...
	[  145.713462]  [<ffffffffa034b1f4>] map_private_extent_buffer+0xd4/0xe0
	[  145.713476]  [<ffffffffa030097a>] ? btrfs_free_path+0x2a/0x40
	[  145.713485]  [<ffffffffa0340864>] btrfs_get_token_64+0x64/0xf0
	[  145.713498]  [<ffffffffa033472c>] relink_extent_backref+0x41c/0x820
	[  145.713508]  [<ffffffffa0334d69>] btrfs_finish_ordered_io+0x239/0xa80

I encounter this warning when running defrag having mkfs.btrfs
with option -M. At the same time there are read/writes & snapshots
running at background.

Signed-off-by: Gui Hecheng <guihc.fnst@cn.fujitsu.com>
Reviewed-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Chris Mason <clm@fb.com>