BPF MAINTAINERS entry "K: bpf" casts too wide a net

[Joe Perches]

The MAINTAINERS entry for BPF (Safe dynamic programs and tools)
includes K: bpf

This entry seems to match far too many files in the kernel sources.

$ git grep --name-only 'bpf' | wc -l
1398

Likely this entry should be removed or improved to ignore unnecessary
matches within files like scripts/checkpatch.pl

Perhaps instead this could be something like:

K:	\bbpf_|_bpf\b