virt: sev-guest: Reduce the scope of SNP command mutex

The SNP command mutex is used to serialize access to the shared buffer,
command handling, and message sequence number.

All shared buffer, command handling, and message sequence updates are done
within snp_send_guest_request(), so moving the mutex to this function is
appropriate and maintains the critical section.

Since the mutex is now taken at a later point in time, remove the lockdep
checks that occur before taking the mutex.

Signed-off-by: Nikunj A Dadhania <nikunj@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Link: https://lore.kernel.org/r/20241009092850.197575-6-nikunj@amd.com