NFS: Clean up locking the nfs_versions list

This patch replaces the nfs_version_mutex and nfs_version_lock with a
single RW lock that protects access to the nfs_versions list.

The mutex around request_module() seemed unnecessary to me, and I
couldn't find any other callers using a lock around calls to
request_module() when I looked.

At the same time, I saw fs/filesystems.c using a RW lock to protect
their filesystems list. This seems like a better idea than a spinlock to
me, so I'm also making that change while I'm here.

Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>