Improve information about logbsize valid values

Improve information about logbsize valid values

Valid values for logbsize depends on whether log_sunit is set
on the filesystem or not and if logbsize is manually set or not.

When manually set, logbsize must be one of the speficied values -
32k to 256k inclusive in power-of-to increments. And, the specified
value must also be a multiple of log_sunit.

The default configuration for v2 logs uses a relaxed restriction,
setting logbsize to log_sunit, independent if it is one of the valid
values or not - also implicitly ignoring the power of two restriction.

Instead of changing valid possible values for logbsize, increasing the
testing matrix and allowing users to use some dubious configuration,
just update the man page to describe this difference in behavior when
manually setting logbsize or leave it to defaults.

This has originally been found by an user attempting to manually set
logbsize to the same value picked by the default configuration just so
to receive an error message as result.

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