fs/affs: make affs exportable

Add standard functions making AFFS work with NFS.

Functions based on ext4 implementation.  Tested on loop device.

Link: http://lkml.kernel.org/r/20170109191208.6085-4-fabf@skynet.be


Signed-off-by: Fabian Frederick <fabf@skynet.be>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>