proto: use bitmask_type for comp flags

proto: use bitmask_type for comp flags

I think flags should be displayed in hexadecimal and should be handled as a
bitmask.

inet/comp.t: WARNING: line: 15: 'nft add rule ip test-ip4 input comp flags 0x00': 'comp flags 0x00' mismatches 'comp flags 0'
inet/comp.t: WARNING: line: 16: 'nft add rule ip test-ip4 input comp flags != 0x23': 'comp flags != 0x23' mismatches 'comp flags != 35'
inet/comp.t: WARNING: line: 17: 'nft add rule ip test-ip4 input comp flags 0x33-0x45': 'comp flags 0x33-0x45' mismatches 'comp flags 51-69'
inet/comp.t: WARNING: line: 18: 'nft add rule ip test-ip4 input comp flags != 0x33-0x45': 'comp flags != 0x33-0x45' mismatches 'comp flags != 51-69'
inet/comp.t: WARNING: line: 19: 'nft add rule ip test-ip4 input comp flags {0x33, 0x55, 0x67, 0x88}': 'comp flags {0x33, 0x55, 0x67, 0x88}' mismatches 'comp flags { 103, 85, 51, 136}'
inet/comp.t: WARNING: line: 21: 'nft add rule ip test-ip4 input comp flags { 0x33-0x55}': 'comp flags { 0x33-0x55}' mismatches 'comp flags { 51-85}'

rfc3173 says that this is unused for future use though.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>