bpf: enable stackmap with build_id in nmi context

Currently, we cannot parse build_id in nmi context because of
up_read(&current->mm->mmap_sem), this makes stackmap with build_id
less useful. This patch enables parsing build_id in nmi by putting
the up_read() call in irq_work. To avoid memory allocation in nmi
context, we use per cpu variable for the irq_work. As a result, only
one irq_work per cpu is allowed. If the irq_work is in-use, we
fallback to only report ips.

Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

init/Kconfig

@@ -1391,6 +1391,7 @@ config BPF_SYSCALL
 	bool "Enable bpf() system call"
 	select ANON_INODES
 	select BPF
+	select IRQ_WORK
 	default n
 	help
 	  Enable the bpf() system call that allows to manipulate eBPF