lttng-modules: update to v2.12.6

lttng-modules: update to v2.12.6

To support building against v5.12+ and v5.13+ kernels, bumping to the
v2.12.6 lttng-release.

We drop the previously backported patches, and bring in the following
commits:

   0d8e4ab9 Version 2.12.6
   78f56ae3 fix: adjust ranges for RHEL 8.2 and 8.3
   4c996ea7 Disable block rwbs bitwise enum in default build
   4ac46085 Disable sched_switch bitwise enum in default build
   5e22c27b Add experimental bitwise enum config option
   937d307e Add defaults to Kconfig options
   4a114d43 Sync `show_inode_state()` macro with upstream stable kernels
   3bd6ac3e fix: block: remove disk_part_iter (v5.12)
   8bbb8c9d Fix: Backport of "Fix: increment buffer offset when failing to copy from user-space"
   30cddf69 Fix: increment buffer offset when failing to copy from user-space
   4733b9ab Sync `show_inode_state()` macro with Ubuntu 4.15 kernel
   ff21ec48 fix: mm, tracing: kfree event name mismatching with provider kmem (v5.12)
   b2b02c29 Set 'stable-2.12' branch in git review config
   00b42dbf fix backport: block: add a disk_uevent helper (v5.12)
   771ff089 fix: Adjust ranges for Ubuntu 5.4.0-67 kernel
   d8933959 fix: block: add a disk_uevent helper (v5.12)
   71034df1 Fix: properly compare type enumeration
   4d879d23 compiler warning cleanup: is_signed_type: compare -1 to 1
   fead3a9c Fix: bytecode linker: validate event and field array/sequence encoding
   92cc3e7f Fix: kretprobe: null ptr deref on session destroy
   49c603ef fix: mm, tracing: record slab name for kmem_cache_free() (v5.12)
   23a2f61f Fix: filter interpreter early-exits on uninitialized value
   b3fdf78b Fix: memory leaks on event destroy

Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>