time: ntp: clean up second_overflow()

Impact: cleanup, no functionality changed

The 'time_adj' local variable is named in a very confusing
way because it almost shadows the 'time_adjust' global
variable - which is used in this same function.

Rename it to 'delta' - to make them stand apart more clearly.

kernel/time/ntp.o:

   text	   data	    bss	    dec	    hex	filename
   2545	    114	    144	   2803	    af3	ntp.o.before
   2545	    114	    144	   2803	    af3	ntp.o.after

md5:
   1bf0b3be564512279ba7cee299d1d2be  ntp.o.before.asm
   1bf0b3be564512279ba7cee299d1d2be  ntp.o.after.asm

Signed-off-by: Ingo Molnar <mingo@elte.hu>