tcp: split tcp_check_space() in two parts

tcp: split tcp_check_space() in two parts

tcp_check_space() is fat and not inlined.

Move its slow path in (out of line) __tcp_check_space()
and make tcp_check_space() an inline function for better TCP performance.

$ scripts/bloat-o-meter -t vmlinux.old vmlinux.new
add/remove: 2/2 grow/shrink: 4/0 up/down: 708/-582 (126)
Function                                     old     new   delta
__tcp_check_space                              -     521    +521
tcp_rcv_established                         1860    1916     +56
tcp_rcv_state_process                       3342    3384     +42
tcp_event_new_data_sent                      248     286     +38
tcp_data_snd_check                            71     106     +35
__pfx___tcp_check_space                        -      16     +16
__pfx_tcp_check_space                         16       -     -16
tcp_check_space                              566       -    -566
Total: Before=24896373, After=24896499, chg +0.00%

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260203050932.3522221-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>