extmod/fsusermount: Expose umount as a public function.

extmod/fsusermount.h

@@ -42,4 +42,5 @@ typedef struct _fs_user_mount_t {
 } fs_user_mount_t;
 
 MP_DECLARE_CONST_FUN_OBJ(fsuser_mount_obj);
+MP_DECLARE_CONST_FUN_OBJ(fsuser_umount_obj);
 MP_DECLARE_CONST_FUN_OBJ(fsuser_mkfs_obj);

py/qstrdefs.h

@@ -671,6 +671,7 @@ Q(status)
 #if MICROPY_FSUSERMOUNT
 // for user-mountable block devices
 Q(mount)
+Q(umount)
 Q(readonly)
 Q(mkfs)
 Q(readblocks)