[SCSI] mpt2sas: add missing allocation check

The __get_free_pages can fail, so the return value should be checked.
Spotted thanks to Stanislaw.

Signed-off-by: Tomas Henzl <thenzl@redhat.com>
Acked-by: "Nandigama, Nagalakshmi" <Nagalakshmi.Nandigama@lsi.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>