bcachefs: fix overflow in fiemap

filefrag (and potentially other utilities that call fiemap) sometimes
pass ULONG_MAX as the length.  fiemap_prep clamps excessively large
lengths - but the calculation of end can overflow if it occurs before
calling fiemap_prep.  When this happens, filefrag assumes it has read to
the end and exits.
Signed-off-by: Reed Riley <reed@riley.engineer>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/fs.c

@@ -964,7 +964,6 @@ static int bch2_fiemap(struct inode *vinode, struct fiemap_extent_info *info,
 	struct btree_iter iter;
 	struct bkey_s_c k;
 	struct bkey_buf cur, prev;
-	struct bpos end = POS(ei->v.i_ino, (start + len) >> 9);
 	unsigned offset_into_extent, sectors;
 	bool have_extent = false;
 	u32 snapshot;
@@ -974,6 +973,7 @@ static int bch2_fiemap(struct inode *vinode, struct fiemap_extent_info *info,
 	if (ret)
 		return ret;
 
+	struct bpos end = POS(ei->v.i_ino, (start + len) >> 9);
 	if (start + len < start)
 		return -EINVAL;