NFSD: fix nfsd_reset_versions for NFSv4.

NFSD: fix nfsd_reset_versions for NFSv4.

[ Upstream commit 800a938f0bf9130c8256116649c0cc5806bfb2fd ]

If you write "-2 -3 -4" to the "versions" file, it will
notice that no versions are enabled, and nfsd_reset_versions()
is called.
This enables all major versions, not no minor versions.
So we lose the invariant that NFSv4 is only advertised when
at least one minor is enabled.

Fix the code to explicitly enable minor versions for v4,
change it to use nfsd_vers() to test and set, and simplify
the code.

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>