mm: add wrapper function for writing word to kernel text space

Adds a function to encapsulate the locking, removal of write-protection,
word write, cache flush and invalidate and restoration
of write protection. This is a convenience function for callers
needing to update a word in kernel text space.

Change-Id: I9832f0ff659ddc62c55819af5318c94b70f5c11c
Signed-off-by: Neil Leeder <nleeder@codeaurora.org>
(cherry picked from commit 32942757bdfb3c67af2cd9c30427adf7d722f7c8)