MIPS: Fix strnlen_user access check

Commit 04324f44 ("MIPS: Remove get_fs/set_fs") changed the access_ok
for strnlen_user to check the whole range, which broke some callers
of strndup_user(). Restore the old behaviour and just check the first byte.

Fixes: 04324f44 ("MIPS: Remove get_fs/set_fs")
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>