Add procedure to review backported btrfs patches to OpenSUSE Leap upstream release

[Hooverdan96]

"We should also document how to retrieve all btrfs patches that have been applied to the vanilla 6.4 kernel our 15.6 OS base carries. This came up in a discussion with @FroggyFlox recently and I believe we have something of the sort from a few years ago on the forum somewhere. That way we enable folks to check for specific fixes they may have heard of in upstream btrfs developments. ... We can then point folks at it when they have specific questions as to the openSUSE kernel we use. And potentially serve as a deterrent to using our Stable kernel backport how-to. ... a good place for this suggestion: where folks can surface their particular kernels backported btrfs patches."

Originally posted by @phillxnet in #515 (comment)

[Hooverdan96]

@FroggyFlox dug up the post referenced above:

https://forum.rockstor.com/t/unresponsive-system/6616/21?u=flox

The command line to see the applied btrfs patches is:

rpm -q --changelog kernel-default | grep btrfs | head -n 40

Choices are:

1. add it to howtos/stable_kernel_backport.rst - suggested by @phillxnet earlier
2. or add it to interface/storage/pools-btrfs.rst