scsi: megaraid: Fix a sleep-in-atomic bug

The driver may sleep under a spin lock, and the function call path is:
mraid_mm_attach_buf (acquire the lock by spin_lock_irqsave)
  pci_pool_alloc(GFP_KERNEL) --> may sleep

To fix it, the "GFP_KERNEL" is replaced with "GFP_ATOMIC".

[mkp: fixed whitespace]

Signed-off-by: Jia-Ju Bai <baijiaju1990@163.com>
Acked-by: Sumit Saxena <sumit.saxena@broadcom.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>