kernel: exportfs: reallow building as a module

It was originally tristate, but was changed to bool to prevent a build
breakage in an earlier patch revision (for a new syscall). The actual
commit[1] changed it in a way this isn't needed anymore, but
accidentially left the EXPORTFS change in. There's no reason for it
not being buildable as a module, so changing it back to tristate is
fine.

see: https://patchwork.kernel.org/patch/812832/

Thank you Jonas Gorski for the patch. 


git-svn-id: svn://svn.openwrt.org/openwrt/trunk@27011 3c298f89-4303-0410-b956-a3cf2f4a3e73

target/linux/generic/patches-2.6.39/030-exportfs_tristate.patch

@@ -0,0 +1,11 @@
+--- a/fs/Kconfig
++++ b/fs/Kconfig
+@@ -47,7 +47,7 @@ config FS_POSIX_ACL
+ 	def_bool n
+ 
+ config EXPORTFS
+-	bool
++	tristate
+ 
+ config FILE_LOCKING
+ 	bool "Enable POSIX file locking API" if EXPERT