fscrypt: Don't use asynchronous CryptoAPI algorithms

fscrypt: Don't use asynchronous CryptoAPI algorithms

Now that fscrypt's incomplete support for non-inline crypto engines has
been removed, and none of the CPU-based algorithms have the
CRYPTO_ALG_ASYNC flag set anymore, there is no need to accommodate
asynchronous algorithms.  Therefore, explicitly allocate only
synchronous algorithms.  Then, remove the code that handled waiting for
asynchronous en/decryption operations to complete.

This commit should *not* be backported to kernels that lack commit
0ba6ec5b2972 ("crypto: x86/aes - stop using the SIMD helper"), as then
it would disable the use of the optimized AES code on x86.

Link: https://lore.kernel.org/r/20250710060754.637098-2-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>