[NETFILTER]: nf_conntrack: avoid duplicate protocol comparison in nf_ct_tuple_equal()

nf_ct_tuple_src_equal() and nf_ct_tuple_dst_equal() both compare the protocol
numbers. Unfortunately gcc doesn't optimize out the second comparison, so
remove it and prefix both functions with __ to indicate that they should not
be used directly.

Saves another 16 byte of text in __nf_conntrack_find() on x86_64:

  nf_conntrack_tuple_taken |  -20 # 320 -> 300, size inlines: 181 -> 161
  __nf_conntrack_find      |  -16 # 267 -> 251, size inlines: 127 -> 115
  __nf_conntrack_confirm   |  -40 # 875 -> 835, size inlines: 570 -> 537
 3 functions changed, 76 bytes removed

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>