fscrypt: split up fscrypt_supported_policy() by policy version

fscrypt: split up fscrypt_supported_policy() by policy version

Make fscrypt_supported_policy() call new functions
fscrypt_supported_v1_policy() and fscrypt_supported_v2_policy(), to
reduce the indentation level and make the code easier to read.

Also adjust the function comment to mention that whether the encryption
policy is supported can also depend on the inode.

No change in behavior.

Link: https://lore.kernel.org/r/20191209211829.239800-2-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>