nfsd: remove unnecessary NULL checks from nfsd_cross_mnt

We can now assume that rqst_exp_get_by_name() does not return NULL; so clean
up some unnecessary checks.

Signed-off-by: "J. Bruce Fields" <bfields@citi.umich.edu>
Acked-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>