generic RDMA READ/WRITE API V5

[Christoph Hellwig <hch@lst.de>]

This series contains patches that implement a first version of a generic
API to handle RDMA READ/WRITE operations as commonly used on the target
(or server) side for storage protocols.

This has been developed for the upcoming NVMe over Fabrics target, and
extensively teѕted as part of that, although this upstream version has
additional updates over the one we're currently using.

It hides details such as the use of MRs for iWarp devices, and will allow
looking at other HCA specifics easily in the future.

This series contains a conversion of the SRP target, and the git tree
below also has a RFC conversion of the iSER target (a little hacky
due to the signature MR support which I can't test)

I think it's basically ready to merge now.

I also have a git tree available at:

	git://git.infradead.org/users/hch/rdma.git rdma-rw-api

Gitweb:

	http://git.infradead.org/users/hch/rdma.git/shortlog/refs/heads/rdma-rw-api

These two also include the RFC iSER target conversion.

Changes since V4:
 - fix SG iteration in rdma_rw_init_mr_wrs
 - address various misc review feedback items from Bart and Leon

Changes since V3:
 - really fold the list_del in mr_pool_get into the right patch

Changes since V2:
 - fold the list_del in mr_pool_get into the right patch
 - clamp the max FR page size length
 - minor srpt style fix
 - spelling fixes
 - renamed rdma_has_read_invalidate to rdma_cap_read_inv

Changes since V1:
 - fixed offset handling in ib_sg_to_pages
 - uses proper SG iterators to handle larger than PAGE_SIZE segments
 - adjusted parameters for some functions to reduce size of the context
 - SRP target support