bcachefs: Improved extent merging

bcachefs: Improved extent merging

Previously, checksummed extents could only be merged when the checksum
covered only the currently live data.

xfstest generic/064 creates a test file, then uses finsert calls to
split the extent, then collapse calls to see if they get merged. But
without any reads to trigger the narrow_crcs path, each of the split
extents will still have a checksum for the entire original extent.

This patch improves the extent merge path so that if either of the
extents we're attempting to merge has a checksum that covers the entire
merged extent, we just use that checksum.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>