btrfs-tools: update to 4.15.1

Drop upstreamed 0001-Fix-build-with-musl-missing-header-include-for-dev_t.patch

Add ftw-subdir-walk.patch as it resolves the RECIPE_NO_UPDATE_REASON.

Add --disable-zstd as libzstd isn't provided in oe-core.

Fix wic testcase, as the minimal fs size is now bigger.

Signed-off-by: Alexander Kanavin <alexander.kanavin@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>