firmware_loader: move firmware sysctl to its own files

Patch series "sysctl: 3rd set of kernel/sysctl cleanups", v2.

This is the third set of patches to help address cleaning the kitchen
seink in kernel/sysctl.c and to move sysctls away to where they are
actually implemented / used.

This patch (of 8):

kernel/sysctl.c is a kitchen sink where everyone leaves their dirty
dishes, this makes it very difficult to maintain.

To help with this maintenance let's start by moving sysctls to places
where they actually belong.  The proc sysctl maintainers do not want to
know what sysctl knobs you wish to add for your own piece of code, we
just care about the core logic.

So move the firmware configuration sysctl table to the only place where
it is used, and make it clear that if sysctls are disabled this is not
used.

[akpm@linux-foundation.org: export register_firmware_config_sysctl and unregister_firmware_config_sysctl to modules]
[akpm@linux-foundation.org: use EXPORT_SYMBOL_NS_GPL instead]
[sfr@canb.auug.org.au...