IB/srp: Get rid of "Target has req_lim 0" messages

It's perfectly valid for a connection to an SRP target to have a
request limit of 0, so get rid of the message about it, which can spam
kernel logs even with printk_ratelimit().  Keep a count of such events
in a "zero_req_lim" SCSI host attribute instead, so someone who cares
can look at the statistics.

Signed-off-by: Roland Dreier <rolandd@cisco.com>