scsi: target: file: Use kzalloc_flex for aio_cmd

scsi: target: file: Use kzalloc_flex for aio_cmd

The target_core_file doesn't initialize the aio_cmd->iocb for the
ki_write_stream. When a write command fd_execute_rw_aio() is executed,
we may get a bogus ki_write_stream value, causing unintended write
failure status when checking iocb->ki_write_stream > max_write_streams
in the block device.

Let's just use kzalloc_flex when allocating the aio_cmd and let
ki_write_stream=0 to fix this issue.

Fixes: 732f25a2895a ("fs: add a write stream field to the kiocb")
Fixes: c27683da6406 ("block: expose write streams for block device nodes")
Cc: stable@vger.kernel.org
Signed-off-by: Thinh Nguyen <Thinh.Nguyen@synopsys.com>
Link: https://patch.msgid.link/f1a2f81c62f043e31f80bb92d5f29893400c8ee2.1773450782.git.Thinh.Nguyen@synopsys.com
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>