net: filter: Fix redefinition warnings on x86-64.

Do not collide with the x86-64 PTRACE user API namespace.

net/core/filter.c:57:0: warning: "R8" redefined [enabled by default]
arch/x86/include/uapi/asm/ptrace-abi.h:38:0: note: this is the location of the previous definition

Fix by adding a BPF_ prefix to the register macros.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: David S. Miller <davem@davemloft.net>