nfsclient: Add a new nfs_client_rdma_enable variable

This patch adds a new nfs_client_rdma_enable variable
to /etc/rc.d/nfsclient to enable the client side of
NFS over RDMA.  The client side of NFS over RDMA
requires the nfsclrdma.ko module, which is still under
test/review.

I wanted to get the "glue" into main so that others
could test the module more easily.  Avaliability of
the module will be announced on freebsd-current@ soon.

It should not affect non-RDMA operation.

I've specified a long MFC, since the module still
requires extensive testing and, hopefully, a review.

MFC after:	3 months

libexec/rc/rc.conf

@@ -384,6 +384,7 @@ automount_flags=""		# Flags to automount(8) (if autofs enabled).
 automountd_flags=""		# Flags to automountd(8) (if autofs enabled).
 autounmountd_flags=""		# Flags to autounmountd(8) (if autofs enabled).
 nfs_client_enable="NO"		# This host is an NFS client (or NO).
+nfs_client_rdma_enable="NO"	# Enable host for NFS over RDMA mounts.
 nfs_access_cache="60"		# Client cache timeout in seconds
 nfs_server_enable="NO"		# This host is an NFS server (or NO).
 nfs_server_flags="-u -t"	# Flags to nfsd (if enabled).

libexec/rc/rc.d/nfsclient

@@ -32,6 +32,9 @@ nfsclient_start()
 			warn "failed to set vfs.nfs.bufpackets"
 		fi
 	fi
+	if checkyesno nfs_client_rdma_enable; then
+		load_kld nfsclrdma || err 1 "Cannot load nfsclrdma"
+	fi
 
 	unmount_all
 }