Revert r295359:

CID 1018688 is a false positive.

The initialization is done by calling vn_start_write(... &mp, flags).
mp is only an output parameter unless (flags & V_MNTREF), and fdesc
doesn't put V_MNTREF in flags.

Pointed out by:	bde

sys/fs/fdescfs/fdesc_vnops.c

@@ -465,7 +465,7 @@ fdesc_setattr(ap)
 {
 	struct vattr *vap = ap->a_vap;
 	struct vnode *vp;
-	struct mount *mp = NULL;
+	struct mount *mp;
 	struct file *fp;
 	struct thread *td = curthread;
 	cap_rights_t rights;