f2fs: Zero f2fs_folio_state on allocation

f2fs: Zero f2fs_folio_state on allocation

f2fs_folio_state is attached to folio->private and is expected to start
with read_pages_pending == 0.  However, the structure was allocated from
ffs_entry_slab without being fully initialized, which can leave
read_pages_pending with stale values.

Allocate the object with __GFP_ZERO so all fields are reliably zeroed at
creation time.

Signed-off-by: Nanzhe Zhao <nzzhao@126.com>
Reviewed-by: Barry Song <baohua@kernel.org>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>