xfs: Fix logbsize validation

An user reported an inconsistency while mounting a V2 log filesystem
with logbsize equals to the stripe unit being used (192k).

The current validation algorithm for the log buffer size enforces the
user to pass a power_of_2 value between [16k-256k].
The manpage dictates the log buffer size must be a multiple of the log
stripe unit, but doesn't specify it must be a power_of_2. Also, if
logbsize is not specified at mount time, it will be set to
max(32768, log_sunit), where log_sunit not necessarily is a power_of_2.

It does seem to me then that logbsize being a power_of_2 constraint must
be relaxed if there is a configured log stripe unit, so this patch
updates the logbsize validation logic to ensure that:

- It can only be set to a specific range [16k-256k]

- Will be aligned to log stripe unit when the latter is set,
  and will be at least the same size as the log stripe unit.

- Enforce it to be power_of_2 aligned when log stripe unit is not set.

This is achieved by factoring out the logbsize validation to a separated
function to avoid a big chain of if conditionals

While at it, update m_logbufs and m_logbsize conditionals in
xfs_fs_validate_params from:
	(x != -1 && x != 0) to (x > 0)

Signed-off-by: Carlos Maiolino <cmaiolino@redhat.com>