ecryptfs: propagate key errors up at mount time

Mounting with invalid key signatures should probably fail, if they were
specifically requested but not available.

Also fix case checks in process_request_key_err() for the right sign of
the errnos, as spotted by Jan Tluka.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Jan Tluka <jtluka@redhat.com>
Acked-by: Michael Halcrow <mhalcrow@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>