xfrm: make xfrm_replay_state_esn_len() return unsigned int

Replay detection bitmaps can't have negative length.

Comparisons with nla_len() are left signed just in case negative value
can sneak in there.

Propagate unsignedness for code size savings:

	add/remove: 0/0 grow/shrink: 0/5 up/down: 0/-38 (-38)
	function                                     old     new   delta
	xfrm_state_construct                        1802    1800      -2
	xfrm_update_ae_params                        295     289      -6
	xfrm_state_migrate                          1345    1339      -6
	xfrm_replay_notify_esn                       349     337     -12
	xfrm_replay_notify_bmp                       345     333     -12

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>