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>

fs/nfs/Kconfig

@@ -79,12 +79,9 @@ config NFS_V4_1
 	depends on NFS_FS && NFS_V4 && EXPERIMENTAL
 	select SUNRPC_BACKCHANNEL
 	select PNFS_FILE_LAYOUT
-	select PNFS_BLOCK
-	select MD
-	select BLK_DEV_DM
 	help
 	  This option enables support for minor version 1 of the NFSv4 protocol
-	  (RFC 5661 and RFC 5663) in the kernel's NFS client.
+	  (RFC 5661) in the kernel's NFS client.
 
 	  If unsure, say N.
 
@@ -92,7 +89,14 @@ config PNFS_FILE_LAYOUT
 	tristate
 
 config PNFS_BLOCK
-	tristate
+	tristate "Provide support for the pNFS Block Layout Driver for NFSv4.1 pNFS (EXPERIMENTAL)"
+	depends on NFS_FS && NFS_V4_1 && BLK_DEV_DM
+	help
+	  Say M here if you want your pNFS client to support the Block Layout Driver
+	  (RFC 5663). Requires Multiple devices driver support (DM) and Device mapper
+	  support (BLK_DEV_DM).
+
+	  If unsure, say N.
 
 config PNFS_OBJLAYOUT
 	tristate "Provide support for the pNFS Objects Layout Driver for NFSv4.1 pNFS (EXPERIMENTAL)"