eCryptfs: Use generic_file_splice_read()

eCryptfs is currently just passing through splice reads to the lower
filesystem.  This is obviously incorrect behavior; the decrypted data is
what needs to be read, not the lower encrypted data.  I cannot think of any
good reason for eCryptfs to implement splice_read, so this patch points the
eCryptfs fops splice_read to use generic_file_splice_read.

Signed-off-by: Michael Halcrow <mhalcrow@us.ibm.com>
Reviewed-by: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>