IB/srp: Remove use_mr argument from srp_map_sg_entry()

Move the srp_map_desc() call from inside srp_map_sg_entry() to
srp_map_sg() such that the use_mr argument can be removed from
srp_map_sg_entry().

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