pNFS: Add nfsstats counters for number of Layouts

For pNFS, Layouts are issued by the server to indicate
where a file's data resides on the DS(s). This patch
adds counters for how many layouts are allocated to
the nfsstatsv1 structure, using two reserved fields.

(cherry picked from commit ce9676de86f267986f61979151101e3d8a6d61bc)