smb: client: flush dirty data before zeroing a range

In the Linux kernel, the following vulnerability has been resolved:

smb: client: flush dirty data before zeroing a range

smb3_zero_range() emulates FALLOC_FL_ZERO_RANGE by invalidating the
page cache over the target range with truncate_pagecache_range() and
then issuing FSCTL_SET_ZERO_DATA to the server.

Dirty data was only flushed conditionally, when the range reached or
extended EOF. For a purely interior zero range that does not reach
EOF, no flush happened, so a dirty folio overlapping the range could
be written back to the server after the FSCTL and refill the range
that was just zeroed with stale data.

Fix this by unconditionally flushing and waiting for dirty data in the
range before invalidating the page cache and issuing
FSCTL_SET_ZERO_DATA, exactly as was done for smb3_punch_hole() in
commit d7d2adcd022b ("smb/client: flush dirty data before punching a
hole"); the two paths are structurally identical here.

This is observed as generic/363 randomly reading stale data where a
zeroed range is expected against Windows Server.