GFS2: revert "GFS2: d_splice_alias() can't return error"

0d0d1107 asserts that "d_splice_alias()
can't return error unless it was given an IS_ERR(inode)".

That was true of the implementation of d_splice_alias, but this is
really a problem with d_splice_alias: at a minimum it should be able to
return -ELOOP in the case where inserting the given dentry would cause a
directory loop.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>