ovl: free stack of paths in ovl_fill_super

This fixes small memory leak after mount.

Kmemleak report:

unreferenced object 0xffff88003683fe00 (size 16):
  comm "mount", pid 2029, jiffies 4294909563 (age 33.380s)
  hex dump (first 16 bytes):
    20 27 1f bb 00 88 ff ff 40 4b 0f 36 02 88 ff ff   '......@K.6....
  backtrace:
    [<ffffffff811f8cd4>] create_object+0x124/0x2c0
    [<ffffffff817a059b>] kmemleak_alloc+0x7b/0xc0
    [<ffffffff811dffe6>] __kmalloc+0x106/0x340
    [<ffffffffa01b7a29>] ovl_fill_super+0x389/0x9a0 [overlay]
    [<ffffffff81200ac4>] mount_nodev+0x54/0xa0
    [<ffffffffa01b7118>] ovl_mount+0x18/0x20 [overlay]
    [<ffffffff81201ab3>] mount_fs+0x43/0x170
    [<ffffffff81220d34>] vfs_kern_mount+0x74/0x170
    [<ffffffff812233ad>] do_mount+0x22d/0xdf0
    [<ffffffff812242cb>] SyS_mount+0x7b/0xc0
    [<ffffffff817b6bee>] entry_SYSCALL_64_fastpath+0x12/0x76
    [<ffffffffffffffff>] 0xffffffffffffffff

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: Miklos Szeredi <miklos@szeredi.hu>
Fixes: a78d9f0d ("ovl: support multiple lower layers")
Cc: <stable@vger.kernel.org> # v4.0+