nfscl: Improve "Consider increasing kern.ipc.maxsockbuf" message

When the setting of kern.ipc.maxsockbuf is less than what is
desired for I/O based on vfs.maxbcachebuf and vfs.nfs.bufpackets,
a console message of "Consider increasing kern.ipc.maxsockbuf".
is printed.

This patch modifies the message to provide a suggested value
for kern.ipc.maxsockbuf.
Note that the setting is only needed when the NFS rsize/wsize
is set to vfs.maxbcachebuf.

While here, make nfs_bufpackets global, so that it can be used
by a future patch that adds a sysctl to set the NFS server's
maximum I/O size. Also, remove "sizeof(u_int32_t)" from the maximum
packet length, since NFS_MAXXDR is already an "overestimate"
of the actual length.

MFC after: 2 weeks