generic RDMA READ/WRITE API V2

[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 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.

Chances 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

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