smb/client: flush dirty data before punching a hole

smb/client: flush dirty data before punching a hole

Punching a hole after a large buffered write may leave the range
reported as data. Reproduce it with:

  xfs_io -f \
    -c "pwrite -b 3m -S 0x61 0 3m" \
    -c "fpunch 1m 1m" \
    -c "seek -h 0" \
    -c "seek -d 1m" \
    /mnt/test/repro

Punching 1 MiB at offset 1 MiB should produce:

  0          1 MiB       2 MiB       3 MiB
  |  DATA    |   HOLE    |   DATA    | EOF

Instead, the entire file is reported as data. SEEK_HOLE(0) returns EOF,
and SEEK_DATA(1M) returns 1M.

This happens because a dirty folio spanning the punched range can be
written back after the punch and refill the hole.

Fix this by flushing and waiting for dirty data in the punched range
before invalidating the page cache and issuing FSCTL_SET_ZERO_DATA.

The xfstests generic/539 pass against Samba/ksmbd with this change.

Signed-off-by: Huiwen He <hehuiwen@kylinos.cn>
Reviewed-by: ChenXiaoSong <chenxiaosong@kylinos.cn>
Signed-off-by: Steve French <stfrench@microsoft.com>