net: filter: x86: split bpf_jit_compile()

Split bpf_jit_compile() into two functions to improve readability
of for(pass++) loop. The change follows similar style of JIT compilers
for arm, powerpc, s390

The body of new do_jit() was not reformatted to reduce noise
in this patch, since the following patch replaces most of it.

Tested with BPF testsuite.

Signed-off-by: Alexei Starovoitov <ast@plumgrid.com>
Signed-off-by: David S. Miller <davem@davemloft.net>