smb: client: Use HMAC-SHA256 library for SMB2 signature calculation

smb: client: Use HMAC-SHA256 library for SMB2 signature calculation

Convert smb2_calc_signature() to use the HMAC-SHA256 library instead of
a "hmac(sha256)" crypto_shash.  This is simpler and faster.  With the
library there's no need to allocate memory, no need to handle errors,
and the HMAC-SHA256 code is accessed directly without inefficient
indirect calls and other unnecessary API overhead.

To make this possible, make __cifs_calc_signature() support both the
HMAC-SHA256 library and crypto_shash.  (crypto_shash is still needed for
HMAC-MD5 and AES-CMAC.  A later commit will switch HMAC-MD5 from shash
to the library.  I'd like to eventually do the same for AES-CMAC, but it
doesn't have a library API yet.  So for now, shash is still needed.)

Also remove the unnecessary 'sigptr' variable.

For now smb3_crypto_shash_allocate() still allocates a "hmac(sha256)"
crypto_shash.  It will be removed in a later commit.

Reviewed-by: Stefan Metzmacher <metze@samba.org>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>