FS_TYPE_END is no longer used as there is no hard-coded limit on the
number of file systems. Replace with the limit used when only in-tree
file systems are enabled.
The test for unsupported file systems is no longer valid. Instead
verify that attempting to register a file system when there is no room
for it fails as expected.
Signed-off-by: Peter A. Bigot <pab@pabigot.com>