[SCSI] megaraid_sas: replace pci_alloc_consitent with dma_alloc_coherent in ioctl path

Replaced pci_alloc_consistent with dma_alloc_coherent from the ioctl path.
This is to avoid situations where ioctl fails for lack of memory
(when system under heavy stress).

Signed-off-by: Sumant Patro <sumant.patro@lsi.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>