NFSD: Move the fh_getattr() helper

Clean up: The fh_getattr() function is part of NFSD's file handle
API, so relocate it.

I've made it an un-inlined function so that trace points and new
functionality can easily be introduced. That increases the size of
nfsd.ko by about a page on my x86_64 system (out of 26MB; compiled
with -O2).

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>