nfsd: allow root to set uid and gid on create

The server silently ignores attempts to set the uid and gid on create.
Based on the comment, this appears to have been done to prevent some
overly-clever IRIX client from causing itself problems.

Perhaps we should remove that hack completely.  For now, at least, it
makes sense to allow root (when no_root_squash is set) to set uid and
gid.

While we're there, since nfsd_create and nfsd_create_v3 share the same
logic, pull that out into a separate function.  And spell out the
individual modifications of ia_valid instead of doing them both at once
inside a conditional.

Thanks to Roger Willcocks <roger@filmlight.ltd.uk> for the bug report
and original patch on which this is based.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>