scsi: st: Replace GFP_ATOMIC with GFP_KERNEL in new_tape_buffer

scsi: st: Replace GFP_ATOMIC with GFP_KERNEL in new_tape_buffer

new_tape_buffer() is never called in atomic context. new_tape_buffer()
is only called by st_probe(), which is only set as ".probe" in struct
scsi_driver.

Despite never getting called from atomic context, new_tape_buffer()
calls kzalloc() with GFP_ATOMIC, which does not sleep for allocation.
GFP_ATOMIC is not necessary and can be replaced with GFP_KERNEL, which
can sleep and improve the possibility of sucessful allocation.

This is found by a static analysis tool named DCNS written by myself.
And I also manually check it.

Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>