[XFS] Fix up the calculation of the reservation overhead to hopefully

[XFS] Fix up the calculation of the reservation overhead to hopefully
include all the components which make up the transaction in the ondisk
log. Having this incomplete has shown up as problems on IRIX when some v2
log changes went in. The symptom was the msg of "xfs_log_write:
reservation ran out. Need to up reservation" and was seen on synchronous
writes on files with lots of holes (and therefore lots of extents).

SGI-PV: 931457
SGI-Modid: xfs-linux:xfs-kern:23095a

Signed-off-by: Tim Shimmin <tes@sgi.com>
Signed-off-by: Nathan Scott <nathans@sgi.com>