For ZFS, change the type of clock_t to int64_t.
For ZFS, change the type of clock_t to int64_t.

The clock_t type in OpenSolaris is long (int64_t on amd64).
On FreeBSD clock_t is int32_t. The clock_t type is used in several places
in the ZFS code to store system uptime in milliseconds ("seconds * hz").

With hz=1000 we have a 32-bit integer overflow in 24 days, 20 hours,
31 minutes and 23.648 seconds. This has a user reported negative impact
on l2arc_feed_thread() and may cause unexpected results from other functions
using clock_t.

Reported by:	Artem Belevich <fbsdlist@src.cx> on freebsd-fs@
MFC after:	1 week

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