This patch allows fscrypto to handle a second key prefix given by filesystem.
The main reason is to provide backward compatibility, since previously f2fs
used "f2fs:" as a crypto prefix instead of "fscrypt:".
Later, ext4 should also provide key_prefix() to give "ext4:".
One concern decribed by Ted would be kinda double check overhead of prefixes.
In x86, for example, validate_user_key consumes 8 ms after boot-up, which turns
out derive_key_aes() consumed most of the time to load specific crypto module.
After such the cold miss, it shows almost zero latencies, which treats as a
Note that request_key() detects wrong prefix in prior to derive_key_aes() even.
Cc: Ted Tso <email@example.com>
Cc: firstname.lastname@example.org # v4.6
Signed-off-by: Jaegeuk Kim <email@example.com>