From: NeilBrown <neilb@cse.unsw.edu.au>

fh_dup2 duplicates an entire svc_fh structure, so in addition to dget()'ing
the dentry, it also needs to bump the reference count on fh_export.

Moved it out of the header file into nfsfh.c for the simple reason that I
couldn't figure out how to reorganize the headers files right so that the
fh_dup2 could use the definition of svc_fh.  Since fh_dup2 is never actually
used outside of nfs4proc.c, it could just as well be moved there....