ARM: net: bpf: move stack documentation

commit 70ec3a6c2c11e4b0e107a65de943a082f9aff351 upstream.

Move the stack documentation towards the top of the file, where it's
relevant for things like the register layout.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>