xfs: factor out a xlog_write_space_advance helper

xfs: factor out a xlog_write_space_advance helper

Add a new xlog_write_space_advance that returns the current place in the
iclog that data is written to, and advances the various counters by the
amount taken from xlog_write_iovec, and also use it xlog_write_partial,
which open codes the counter adjustments, but misses the asserts.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>