exportfs: reallow building as a module

Commit 990d6c2d ("vfs: Add name to file
handle conversion support") changed EXPORTFS to be a bool.
This was needed for earlier revisions of the original patch, but the actual
commit put the code needing it into its own file that only gets compiled
when FHANDLE is selected which in turn selects EXPORTFS.
So EXPORTFS can be safely compiled as a module when not selecting FHANDLE.
Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Acked-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

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