nfsd: clean up mnt_want_write calls

Multiple mnt_want_write() calls in the switch statement looks really
ugly.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Acked-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>