megaraid: Fix possible NULL pointer deference in mraid_mm_ioctl

megaraid: Fix possible NULL pointer deference in mraid_mm_ioctl

Orabug: 22620626

This adds the needed check after the call to the function
mraid_mm_alloc_kioc in order to make sure that this function has not
returned NULL and therefore makes sure we do not deference a NULL
pointer if one is returned by mraid_mm_alloc_kioc.  Further more add
needed comments explaining that this function call can return NULL if
the list head is empty for the pointer passed in order to allow furture
users to understand this required pointer check.

Signed-off-by: Nicholas Krause <xerofoify@gmail.com>
Acked-by: Sumit Saxena <sumit.saxena@avagotech.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
(cherry picked from commit 7296f62f0322d808362b21064deb34f20799c20d)
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>