rpcrdma: Merge svcrdma and xprtrdma modules into one

Bi-directional RPC support means code in svcrdma.ko invokes a bit of
code in xprtrdma.ko, and vice versa. To avoid loader/linker loops,
merge the server and client side modules together into a single
module.

When backchannel capabilities are added, the combined module will
register all needed transport capabilities so that Upper Layer
consumers automatically have everything needed to create a
bi-directional transport connection.

Module aliases are added for backwards compatibility with user
space, which still may expect svcrdma.ko or xprtrdma.ko to be
present.

This commit reverts commit 2e8c12e1

 ("xprtrdma: add separate
Kconfig options for NFSoRDMA client and server support") and
provides a single CONFIG option for enabling the new module.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>