nfsd: allow reaping files still under writeback

On most filesystems, there is no reason to delay reaping an nfsd_file
just because its underlying inode is still under writeback. nfsd just
relies on client activity or the local flusher threads to do writeback.

The main exception is NFS, which flushes all of its dirty data on last
close. Add a new EXPORT_OP_FLUSH_ON_CLOSE flag to allow filesystems to
signal that they do this, and only skip closing files under writeback on
such filesystems.

Also, remove a redundant NULL file pointer check in
nfsd_file_check_writeback, and clean up nfs's export op flag
definitions.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Acked-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>