xfs: use better names for size members in xfs_log_vec

xfs: use better names for size members in xfs_log_vec

The lv_size member counts the size of the entire allocation, rename it to
lv_alloc_size to make that clear.

The lv_buf_len member tracks how much of lv_buf has been used up
to format the log item, rename it to lv_buf_used to make that more clear.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>