fix ufs write vs readpage race when writing into a hole

Followup to the UFS series - with the way we clear the new blocks (via
buffer cache, possibly on more than a page worth of file) we really
should not insert a reference to new block into inode block tree until
after we'd cleared it.
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

fs/ufs/balloc.c

@@ -417,14 +417,14 @@ u64 ufs_new_fragments(struct inode *inode, void *p, u64 fragment,
 	if (oldcount == 0) {
 		result = ufs_alloc_fragments (inode, cgno, goal, count, err);
 		if (result) {
+			ufs_clear_frags(inode, result + oldcount,
+					newcount - oldcount, locked_page != NULL);
 			write_seqlock(&UFS_I(inode)->meta_lock);
 			ufs_cpu_to_data_ptr(sb, p, result);
 			write_sequnlock(&UFS_I(inode)->meta_lock);
 			*err = 0;
 			UFS_I(inode)->i_lastfrag =
 				max(UFS_I(inode)->i_lastfrag, fragment + count);
-			ufs_clear_frags(inode, result + oldcount,
-					newcount - oldcount, locked_page != NULL);
 		}
 		mutex_unlock(&UFS_SB(sb)->s_lock);
 		UFSD("EXIT, result %llu\n", (unsigned long long)result);