ocfs2: fix d_splice_alias() return code checking

d_splice_alias() can return a valid dentry, NULL or an ERR_PTR.
Currently the code checks not for ERR_PTR and will cuase an oops in
ocfs2_dentry_attach_lock().  Fix this by using IS_ERR_OR_NULL().

Signed-off-by: Richard Weinberger <richard@nod.at>
Cc: Mark Fasheh <mfasheh@suse.com>
Cc: Joel Becker <jlbec@evilplan.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>