ANDROID: block: make blk_crypto_start_using_mode() properly check for support

If blk-crypto-fallback is needed but is disabled by kconfig, make
blk_crypto_start_using_mode() return an error rather than succeeding.
Use ENOPKG, which matches the error code used by fscrypt when crypto API
support is missing with fs-layer encryption.

Also, if blk-crypto-fallback is needed but the algorithm is missing from
the kernel's crypto API, change the error code from ENOENT to ENOPKG.

This is needed for VtsKernelEncryptionTest to pass on some devices.

Bug: 137270441
Bug: 151100202
Test: 'atest vts_kernel_encryption_test' on Pixel 4 with the
      inline crypto patches backported, and also on Cuttlefish.
Change-Id: Iedf00ca8e48c74a5d4c40b12712f38738a04ef11
Signed-off-by: Eric Biggers <ebiggers@google.com>