checkpatch: do not apply "initialise globals to 0" check to BPF progs

checkpatch: do not apply "initialise globals to 0" check to BPF progs

BPF programs explicitly initialise global variables to 0 to make sure
clang (v10 or older) do not put the variables in the common section.  Skip
"initialise globals to 0" check for BPF programs to elimiate error
messages like:

    ERROR: do not initialise globals to 0
    #19: FILE: samples/bpf/tracex1_kern.c:21:

Link: https://lkml.kernel.org/r/20210209211954.490077-1-songliubraving@fb.com
Signed-off-by: Song Liu <songliubraving@fb.com>
Acked-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>