IB/srp: Add SRP_CRED_REQ support

This series of three patches adds SRP_CRED_REQ support in ib_srp, which is a feature defined in the SRP (draft) standard.

Changes in v4 compared to v3:
- Dropped the fourth patch since it has been merged.
- Introduced the symbolic constant SRP_TSK_MGMT_RSV, which represents the number of slots reserved in the tx ring for SRP task management requests.
- One slot of the transmit ring buffer is now reserved for sending SRP_TSK_MGMT requests.
- The BUILD_BUG_ON_NOT_POWER_OF_2() macro is now used to verify whether SRP_RQ_MASK and SRP_SQ_MASK really are bitmasks.
- Rearranged function order such that no new forward declarations have to be introduced in the second patch.
- Eliminated the srp_target_port.tx_req and tx_rsp variables.
- Removed the SRP_OP_RSP flag.
- Added a source code comment that explains why can_queue must be less than req_lim.

Changes in v3 compared to v2:
- Corrected completion queue size from SRP_REQ_SQ_SIZE (63) into SRP_SQ_SIZE (64).
- Merged the two functions __srp_get_tx_req_iu() and __srp_get_tx_rsp_iu() into a single function __srp_get_tx_iu().
- Removed __attribute__((packed)) from the structure definitions added in include/scsi/srp.h because it was superfluous.
- Modified descriptions of patches 1/4, 2/4 and 3/4.

Changes in v2 compared to v1:
- Split the big patch into multiple patches, which makes reviewing easier.
- Instead of defining separate queues for transmitting requests and responses, a single queue is now used.
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