libsupport: use explicit type widths instead of time_t

libsupport: use explicit type widths instead of time_t

The in-memory data structures used time_t for the grace period (which
is a delta timestamp denominated in seconds), as well as the soft
limit expiration time (which is an actual time_t).  Use an explicit
__u32 for the former, and the __u64 for the latter.

This silences a Coverity warning, but more importantly, using an
explicit __u64 for the expiration time means that running e2fsck on a
platform with a 32-bit time_t, and it needs to read and then modify a
quota structure, we won't lose the high 32-bits of the quota
expiration time.

Addresses-Coverity-Bug: 1531824
Signed-off-by: Theodore Ts'o <tytso@mit.edu>