scsi: mpi3mr: Avoid memcpy field-spanning write WARNING

When the "storcli2 show" command is executed for eHBA-9600, mpi3mr driver
prints this WARNING message:

  memcpy: detected field-spanning write (size 128) of single field "bsg_reply_buf->reply_buf" at drivers/scsi/mpi3mr/mpi3mr_app.c:1658 (size 1)
  WARNING: CPU: 0 PID: 12760 at drivers/scsi/mpi3mr/mpi3mr_app.c:1658 mpi3mr_bsg_request+0x6b12/0x7f10 [mpi3mr]

The cause of the WARN is 128 bytes memcpy to the 1 byte size array "__u8
replay_buf[1]" in the struct mpi3mr_bsg_in_reply_buf. The array is intended
to be a flexible length array, so the WARN is a false positive.

To suppress the WARN, remove the constant number '1' from the array
declaration and clarify that it has flexible length. Also, adjust the
memory allocation size to match the change.

Suggested-by: Sathya Prakash Veerichetty <sathya.prakash@broadcom.com>
Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Link: https://lore.kernel.org/r/20240323084155.166835-1-shinichiro.kawasaki@wdc.com
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>