IB/srp: Use kstrtoull() instead of simple_strtoull()

Use kstrtoull() since simple_strtoull() is deprecated. This patch
improves error checking but otherwise does not change any functionality.

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>