xfs: properly account xfs_buf items as reclaimable

It was reported that xfs_buf items are not accounted for as
reclaimable, and therefore various statistics in /proc/meminfo
and /proc/vmstat do not reflect their reclaimable nature as
other slab-allocated items with shrinkers would do, which in
turn affects the accuracy of stats such as MemAvailable.

2 patches here to add accounting for xfs_bufs allocated via
xfs_buf_alloc_kmem() and via xfs_buf_alloc_folio().

I was not sure how to handle xfs_buf_alloc_vmalloc but I think
that's a relatively rare path, and the above two will capture
the majority of xfs_buf allocations.

I've cc:d linux-mm just because i think this is the first
user of NR_KERNEL_MISC_RECLAIMABLE in the tree?

Thanks,
-Eric