ext4: Change unsigned long to unsigned int

Convert the unsigned longs that are most responsible for bloating the
stack usage on 64-bit systems.

Nearly all places in the ext3/4 code which uses "unsigned long" is
probably a bug, since on 32-bit systems a ulong a 32-bits, which means
we are wasting stack space on 64-bit systems.

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