common/encrypt: allow the use of 'fscrypt:' as key prefix

common/encrypt: allow the use of 'fscrypt:' as key prefix

fscrypt keys have used the $FSTYP as prefix.  However this format is being
deprecated and newer kernels are expected to use the generic 'fscrypt:'
prefix instead.  This patch adds support for this new prefix, and only
uses $FSTYP on filesystems that didn't initially supported it, i.e. ext4 and
f2fs.  This will allow old kernels to be tested.

Signed-off-by: Luís Henriques <lhenriques@suse.de>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>