nfsd: Fix Verify for attributes like FilesAvail

When the Verify operation calls nfsv4_loadattr(), it provides
the "struct statfs" information that can be used for doing a
compare for FilesAvail, FilesFree, FilesTotal, SpaceAvail,
SpaceFree and SpaceTotal. However, the code erroneously
used the "struct nfsstatfs *" argument that is NULL.
This patch fixes these cases to use the correct argument
structure. For the case of FilesAvail, the code in
nfsv4_fillattr() was factored out into a separate function
called nfsv4_filesavail(), so that it can be called from
nfsv4_loadattr() as well as nfsv4_fillattr().

In fact, most of the code in nfsv4_filesavail() is old
OpenBSD code that does not build/run on FreeBSD, but I
left it in place, in case it is of some use someday.

I am not aware of any extant NFSv4 client that does Verify
on these attributes.

Reported by: rtm@lcs.mit.edu
Tested by: rtm@lcs.mit.edu
PR: 260176
MFC after: 2 weeks