xfs: rearrange code in xfs_buf_item.c

xfs: rearrange code in xfs_buf_item.c

The code to initialise, release and free items is all the way down
the bottom of the file. Upcoming fixes need to these functions
earlier in the file, so move them to the top.

There is one code change in this move - the parameter to
xfs_buf_item_relse() is changed from the xfs_buf to the
xfs_buf_log_item - the thing that the function is releasing.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>