xfs: disambiguate units for ftrace fields tagged "len"

Some of our tracepoints have a field known as "len".  That name doesn't
describe any units, which makes the fields not very useful.  Rename the
fields to capture units and ensure the format is hexadecimal.

"fsbcount" are in units of fs blocks
"bbcount" are in units of 512b blocks
"ireccount" are in units of inodes

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>