mptfusion: use memdup_user

Let memdup_user handle the kmalloc, copy_from_user and error checking
kfree code.

Spotted by the following smatch (false positive) warning:

  drivers/message/fusion/mptctl.c:1369 mptctl_getiocinfo() warn:
    possible info leak 'karg'

Signed-off-by: Joe Lawrence <joe.lawrence@stratus.com>
Acked-by: Sreekanth Reddy <Sreekanth.Reddy@avagotech.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>