nfsd: add some comments to nfsd_file_do_acquire

David Howells mentioned that he found this bit of code confusing, so
sprinkle in some comments to clarify.

Reported-by: David Howells <dhowells@redhat.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>