amd64: only check for error != 0 in the inlined part of l1d flush check

this replaces the following near the syscall exit:
cmp $0x39,%rax
ja 0xffffffff8108f82c
movabs $0x200001800060005,%rcx
bt %rax,%rcx
jae 0xffffffff8108f82c

with:
test %edi,%edi
jne 0xffffffff8091a49c