fs/affs: add prefix to some functions

fs/affs: add prefix to some functions

secs_to_datestamp(time64_t secs, struct affs_date *ds);
prot_to_mode(u32 prot);
mode_to_prot(struct inode *inode);

were declared without affs_ prefix

Link: http://lkml.kernel.org/r/20170109191208.6085-6-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>