NFS41: make PNFS_BLOCK selectable

PNFS_BLOCK needs BLK_DEV_DM/MD, which is not a dependency for other
pnfs layout drivers. Seperate it out so others can still build when
BLK_DEV_DM/MD is not enabled.

Also change select to depends on to avoid build failures.

Reported-and-tested-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Peng Tao <peng_tao@emc.com>
Acked-by: Benny Halevy <bhalevy@tonian.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>