bpfilter: document build requirements for bpfilter_umh

It's not intuitively obvious that bpfilter_umh is a statically linked binary.
Mention the toolchain requirement in the Kconfig help, so people
have an easier time figuring out what's needed.
Signed-off-by: Valdis Kletnieks <valdis.kletnieks@vt.edu>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

net/bpfilter/Kconfig

@@ -13,4 +13,8 @@ config BPFILTER_UMH
 	default m
 	help
 	  This builds bpfilter kernel module with embedded user mode helper
+
+	  Note: your toolchain must support building static binaries, since
+	  rootfs isn't mounted at the time when __init functions are called
+	  and do_execv won't be able to find the elf interpreter.
 endif