IB/srp: Introduce srp_device.use_fmr

Introduce the variable srp_device.use_fmr. Leave out the dev->has_fr /
dev->has_fmr and ch->fr_pool / ch->fmr_pool checks since these are
redundant. This patch does not change any functionality but makes the
source code easier to read.

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