IB/srp: Fix IPv6 address parsing

Split IPv6 addresses at the colon that separates the IPv6 address
and the port number instead of at a colon in the middle of the IPv6
address. Check whether the IPv6 address is surrounded with square
brackets.

Fixes: 19f31343 ("IB/srp: Add RDMA/CM support")
Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>