[pnfs] pnfs-gfs2 layout cache

[David M. Richter]

	hello, benny,

	following your suggestion, i ginned up a slab cache for the gfs2 layout
allocations.  i tested this and it appears fine.

	i had initially put most of the cache stuff along with the rest of the gfs2
slabs, but i ended up having to include a lot of other nfs headers and the 
extra ifdefs were kind of an eyesore.  so i don't know if this fits
stylistically, but here's a stab at it.

	thanks,

	d
	.