libext2fs: Make icount use a 32-bit counter

libext2fs: Make icount use a 32-bit counter

In order to more accurately count the number of directories, which
with the DIR_NLINKS feature can now be greater than 65,000, change
icount to use a 32-bit counter.  This doesn't cost us anything extra
when the icount data structures are stored in memory, since due to
padding for alignment reasons.

If the actual count is greater than 65,500, we return 65,500.  This is
because e2fsck doesn't actually need to know the exact count; it only
needs to know if the number of subdirectories is greater than 65,000.
In the future if someone really needs to know the exact number, we
could add a 32-bit interface.  One isn't needed now, though.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>