Replace mbuf macros with the code they would generate in the NFS code.

When the code was ported to Mac OS/X, mbuf handling functions were
converted to using the Mac OS/X accessor functions. For FreeBSD, they
are a simple set of macros in sys/fs/nfs/nfskpiport.h.
Since porting to Mac OS/X is no longer a consideration, replacement of
these macros with the code generated by them makes the code more
readable.
When support for external page mbufs is added as needed by the KERN_TLS,
the patch becomes simpler if done without the macros.

This patch should not result in any semantic change.
This conversion will be committed one file at a time.