f2fs: fix KMSAN uninit-value in extent_info usage

KMSAN reported a use of uninitialized value in `__is_extent_mergeable()`
 and `__is_back_mergeable()` via the read extent tree path.

The root cause is that `get_read_extent_info()` only initializes three
fields (`fofs`, `blk`, `len`) of `struct extent_info`, leaving the
remaining fields uninitialized. This leads to undefined behavior
when those fields are accessed later, especially during
extent merging.

Fix it by zero-initializing the `extent_info` struct before population.

Reported-by: [email protected]
Closes: https://syzkaller.appspot.com/bug?extid=b8c1d60e95df65e827d4
Fixes: 94afd6d ("f2fs: extent cache: support unaligned extent")
Reviewed-by: Chao Yu <[email protected]>
Signed-off-by: Abinash Singh <[email protected]>
Signed-off-by: Jaegeuk Kim <[email protected]>

Author: AbinashLalotra

fs/f2fs/extent_cache.c

@@ -414,7 +414,7 @@ void f2fs_init_read_extent_tree(struct inode *inode, struct folio *ifolio)
 	struct f2fs_extent *i_ext = &F2FS_INODE(&ifolio->page)->i_ext;
 	struct extent_tree *et;
 	struct extent_node *en;
-	struct extent_info ei;
+	struct extent_info ei = {0};
 
 	if (!__may_extent_tree(inode, EX_READ)) {
 		/* drop largest read extent */