fscrypt: name all function parameters

Name all the function parameters.  This makes it so that we don't have a
mix of both styles, so it won't be ambiguous what to use in new fscrypt
patches.  This also matches the checkpatch expectation.

Link: https://lore.kernel.org/r/20200511191358.53096-3-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>