ubifs: don't bother checking for encryption key in ->mmap()

Since only an open file can be mmap'ed, and we only allow open()ing an
encrypted file when its key is available, there is no need to check for
the key again before permitting each mmap().

Signed-off-by: Eric Biggers <ebiggers@google.com>
Acked-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Richard Weinberger <richard@nod.at>