ubifs: Fix inconsistent inode size when powercut happens during appendant writing

UBIFS always make sure that the data length won't beyond the inode size
by writing inode before writing page(See ubifs_writepage.). After commit
c35acef3("ubifs: Convert ubifs_writepage to use a folio"),
the rule is broken in one case: Given a file with size 3, then write 4096
from the offset 0, following process will make inode size be smaller than
file data length after powercut & recovery:
         P1             P2
ubifs_writepage
 len = folio_size(folio) // 4096
 if (folio_pos(folio) + len <= i_size) // condition 1: 0 + 4096 <= 4096
		          //(i_size is updated as 4096 in ubifs_write_end)
   if (folio_pos(folio) >= synced_i_size) // condition 2: 0 >= 3, false
      write_inode // Skipped, because condition 2 is false
   do_writepage(folio, len) // write one page

		do_commit // data node won't be replayed in next mounting
 >> Powercut <<

So, inode size(4096) is not updated into disk, we will get following
error messages in next mounting(chk_fs = 1):
 check_leaf [ubifs]: data node at LEB 14:2048 is not within inode size 3
 dbg_walk_index [ubifs]: leaf checking function returned error -22, for
 leaf at LEB 14:2048

Fix it by modifying condition 2 as original comparison(Compare the page
index of synced_i_size with current page index).

Fixes: c35acef3 ("ubifs: Convert ubifs_writepage to use a folio")
Link: https://bugzilla.kernel.org/show_bug.cgi?id=218934


Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>