fsck: remove a few statics

fsck: remove a few statics

function                                             old     new   delta
kill_all_if_got_signal                                 -      60     +60
notitle                                                1       -      -1
doall                                                  1       -      -1
cancel_requested                                       1       -      -1
num_devices                                            4       -      -4
devices                                                4       -      -4
signal_cancel                                          8       -      -8
fsck_main                                           1893    1870     -23
kill_all_if_cancel_requested                          60       -     -60
------------------------------------------------------------------------------
(add/remove: 1/7 grow/shrink: 0/1 up/down: 60/-102)           Total: -42 bytes
   text    data     bss     dec     hex filename
 824205     458    6956  831619   cb083 busybox_old
 824174     458    6948  831580   cb05c busybox_unstripped

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>