CVE-2026-98684: smb: client: flush and commit data before querying allocated ranges

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

smb: client: flush and commit data before querying allocated ranges

The mode 0 / FALLOCFLKEEPSIZE fallocate emulation for small interior ranges in smb3simplefallocaterange() issues FSCTLQUERYALLOCATEDRANGES and zero-fills any sub-range the server reports as an unallocated hole, to force block allocation without changing file contents.

This is unsafe against Windows servers. Per the Win32 documentation, FSCTLQUERYALLOCATEDRANGES only reports ranges that may contain nonzero data and is explicitly not coherent with recently written data:

"a call to FSCTLQUERYALLOCATEDRANGES [after writing to a network file] would not necessarily return a correct list of allocated regions. To ensure coherency [...] flush the data to the file"

fsx (generic/363) reproduces the resulting corruption against Windows Server 2022: a range is written, an uncached read returns the data, yet the next FSCTLQUERYALLOCATEDRANGES on that range reports it as a whole hole, so the emulation overwrites live data with zeroes. A network trace confirmed the WRITE, the READ returning data and the query returning an empty range list within microseconds. Samba does not exhibit this; its query is coherent with the written data.

Write back the dirty pages, drain any outstanding I/O and issue an SMB2 FLUSH to force the server to commit the data before querying the allocated ranges, so the query reflects the data actually present.

With this, generic/363 passes against both Windows Server 2022 and Samba over 100000 fsx operations.