ib_srpt: use kvmalloc to allocate ring pointers

An array of pointers to SRPT contexts in ib_device is over 30KiB even
in default case, in which an amount of contexts is 4095. The patch
is intended to weed out large contigous allocation for non-DMA memory.

Signed-off-by: Jan Dakinevich <jan.dakinevich@virtuozzo.com>
Reviewed-by: Bart Van Assche <bart.vanassche@wdc.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>