MAINTAINERS: add an entry for MM BPF extensions

MAINTAINERS: add an entry for MM BPF extensions

Let's create a separate entry for MM BPF extensions: these patches
often require an attention from both bpf and mm communities.

Signed-off-by: Roman Gushchin <roman.gushchin@linux.dev>
Link: https://lore.kernel.org/r/20251223044156.208250-7-roman.gushchin@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>