kmod/core: Support live patching on NMI handlers

Support live patching on NMI handlers. This adds checks for
possible inconsistency of live patching on NMI handlers.
The inconsistency problem means that any concurrent execution
of old function and new function, which can lead unexpected results.

Current kpatch checks possible inconsistency problem with
stop_machine, which can cover only threads and normal interrupts.
However, beacuse NMI can not stop with it, stop_machine is not
enough for live patching on NMI handlers or sub-functions which are
invoked in the NMI context.

To check for possible inconsistency of live patching on those
functions, add an atomic flag to count patching target functions
invoked in NMI context while updating kpatch hash table. If the
flag is set by the target functions in NMI, we can not ensure
there is no concurrent execution on it.

This fixes the issue #65.

Changes from v3:
 - Fix kpatch_apply/remove_patch to return 0 if succeeded.

Changes from v2:
 - Clean up kpatch_get_committed_func as same style of kpatch_get_func.
 - Rename opr to op in kpatch_ftrace_handler.
 - Consolidate in_nmi() and kpatch_operation check into one condition.
 - Fix UNPATCH/PATCH mistype in kpatch_register.

Changes from v1:
 - Rename inconsistent_flag to kpatch_status.
 - Introduce new enums and helper functions for kpatch_status.
 - Use hash_del_rcu instead of hlist_del_rcu.
 - Rename get_committed_func to kpatch_get_committed_func.
 - Use ACCESS_ONCE for kpatch_operation to prevent compiler optimization.
 - Fix to remove (!func || func->updating) condition from NMI check.
 - Add more precise comments.
 - Fix setting order of kpatch_status and kpatch_operation.

Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>