mm, vmstat: remove the NR_WRITEBACK_TEMP node_stat_item counter
The only user of the counter (FUSE) was removed in commit 0c58a97f919c
("fuse: remove tmp folio for writebacks and internal rb tree") so follow
the established pattern of removing the counter and hardcoding 0 in
meminfo output, as done recently with NR_BOUNCE. Update documentation
for procfs, including for the value for Bounce that was missed when
removing its counter.
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>