f2fs: fix to zero post-eof page

fstest reports a f2fs bug:

generic/363 42s ... [failed, exit status 1]- output mismatch (see /share/git/fstests/results//generic/363.out.bad)
    --- tests/generic/363.out   2025-01-12 21:57:40.271440542 +0800
    +++ /share/git/fstests/results//generic/363.out.bad 2025-05-19 19:55:58.000000000 +0800
    @@ -1,2 +1,78 @@
     QA output created by 363
     fsx -q -S 0 -e 1 -N 100000
    +READ BAD DATA: offset = 0xd6fb, size = 0xf044, fname = /mnt/f2fs/junk
    +OFFSET      GOOD    BAD     RANGE
    +0x1540d     0x0000  0x2a25  0x0
    +operation# (mod 256) for the bad data may be 37
    +0x1540e     0x0000  0x2527  0x1
    ...
    (Run 'diff -u /share/git/fstests/tests/generic/363.out /share/git/fstests/results//generic/363.out.bad'  to see the entire diff)
Ran: generic/363
Failures: generic/363
Failed 1 of 1 tests

The root cause is user can update post-eof page via mmap [1], however, f2fs
missed to zero post-eof page in below operations, so, once it expands i_size,
then it will include dummy data locates previous post-eof page, so during
below operations, we need to zero post-eof page.

Operations which can include dummy data after previous i_size after expanding
i_size:
- write
- mapwrite [1]
- truncate
- fallocate
 * preallocate
 * zero_range
 * insert_range
 * collapse_range
- clone_range (doesn’t support in f2fs)
- copy_range (doesn’t support in f2fs)

[1] https://man7.org/linux/man-pages/man2/mmap.2.html

 'BUG section'

Cc: stable@kernel.org
Signed-off-by: Chao Yu <chao@kernel.org>
Reviewed-by: Zhiguo Niu <zhiguo.niu@unisoc.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>