Merge PR #24229 into master

Merge PR #24229 into master

* refs/pull/24229/head:
	common: drop BL_BACKWARD_COMPAT in bufferlist.
	osd: OSDMap encoding uses bufferlist::contiguous_filler.
	mds: encode_xattrs() uses buffer::list::contiguous_filler.
	common: introduce contiguous_filler to optimize ENCODE_START.
	common: optimize hole appending in bufferlist.
	common: duplicate an encoding macro to suppress warnings.
	common: drop backward iteration from bufferlist.

Reviewed-by: Gregory Farnum <gfarnum@redhat.com>
Reviewed-by: Josh Durgin <jdurgin@redhat.com>