new helpers: fh_{want,drop}_write()

A bunch of places in nfsd does mnt_{want,drop}_write on vfsmount of
export of given fhandle.  Switched to obvious inlined helpers...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/nfsd/nfs4proc.c b/fs/nfsd/nfs4proc.c
index fa38336..c5e28ed8 100644
--- a/fs/nfsd/nfs4proc.c
+++ b/fs/nfsd/nfs4proc.c
@@ -838,7 +838,7 @@
 			return status;
 		}
 	}
-	status = mnt_want_write(cstate->current_fh.fh_export->ex_path.mnt);
+	status = fh_want_write(&cstate->current_fh);
 	if (status)
 		return status;
 	status = nfs_ok;
@@ -856,7 +856,7 @@
 	status = nfsd_setattr(rqstp, &cstate->current_fh, &setattr->sa_iattr,
 				0, (time_t)0);
 out:
-	mnt_drop_write(cstate->current_fh.fh_export->ex_path.mnt);
+	fh_drop_write(&cstate->current_fh);
 	return status;
 }