NFSv4.1: Fix the r/wsize checking

The intention of nfs4_session_set_rwsize() was to cap the r/wsize to the
buffer sizes negotiated by the CREATE_SESSION. The initial code had a
bug whereby we would not check the values negotiated by nfs_probe_fsinfo()
(the assumption being that CREATE_SESSION will always negotiate buffer values
that are sane w.r.t. the server's preferred r/wsizes) but would only check
values set by the user in the 'mount' command.

The code was changed in 4.11 to _always_ set the r/wsize, meaning that we
now never use the server preferred r/wsizes. This is the regression that
this patch fixes.
Also rename the function to nfs4_session_limit_rwsize() in order to avoid
future confusion.

Fixes: 03385332 (NFSv4.1 respect server's max size in CREATE_SESSION")
Cc: stable@vger.kernel.org # v4.11+
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>