pstore: Allow the user to explicitly choose a backend

pstore only allows one backend to be registered at present, but the
system may provide several. Add a parameter to allow the user to choose
which backend will be used rather than just relying on load order.

Signed-off-by: Matthew Garrett <mjg@redhat.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>

Documentation/kernel-parameters.txt

@@ -2151,6 +2151,8 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			[HW,MOUSE] Controls Logitech smartscroll autorepeat.
 			0 = disabled, 1 = enabled (default).
 
+	pstore.backend=	Specify the name of the pstore backend to use
+
 	pt.		[PARIDE]
 			See Documentation/blockdev/paride.txt.