lib/base64: RFC4648-compliant base64 encoding

Add RFC4648-compliant base64 encoding and decoding routines, based on
the base64url encoding in fs/crypto/fname.c.

Signed-off-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Himanshu Madhani <himanshu.madhani@oracle.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Cc: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>