fscrypt: use ENOPKG when crypto API support missing

Return ENOPKG rather than ENOENT when trying to open a file that's
encrypted using algorithms not available in the kernel's crypto API.

This avoids an ambiguity, since ENOENT is also returned when the file
doesn't exist.

Note: this is the same approach I'm taking for fs-verity.

Signed-off-by: Eric Biggers <ebiggers@google.com>