xfs: refactor extent validation for 5.11

While reviewing the "strengthen log intent validation" series, Brian
Foster suggested that we should refactor all the validation code that
checks physical extents into a common helper, so I've done that for both
data and rt volume mappings.  I also did the same for file ranges, since
those were kind of a mess of open-coded stuff.  Dave Chinner also asked
to rename xfs_fc -> xfs_fs so that's the last patch.