os/LFNIndex: remove alt xattr after unlink

os/LFNIndex: remove alt xattr after unlink

After we unlink, if the nlink on the inode is still non-zero, remove the
alt xattr.  We can *only* do this after the rename or unlink operation
because we don't want to leave a file system link in place without the
matching xattr; hence the fsync_dir() call.

Note that this might leak an alt xattr if we happen to fail after the
rename/unlink but before the removexattr is committed.  We'll fix that
next.

Signed-off-by: Sage Weil <sage@redhat.com>