Emulate illumos interface cv_timedwait_hires()

Needed for Illumos #3582. This interface is supposed to support
a variable-resolution timeout with nanosecond granularity.  This
implementation rounds up to microsecond resolution, as nanosecond-
precision timing is rarely needed for real-world performance
tuning and may incur unnecessary busy-waiting.  usleep_range() is
used if available, otherwise udelay() or msleep() are used
depending on the length of the delay interval.

Add flags from sys/callo.h as these are used to control the behavior of
cv_timedwait_hires().  Specifically,

CALLOUT_FLAG_ABSOLUTE
    Normally, the expiration passed to the timeout API functions is
    an expiration interval. If this flag is specified, then it is
    interpreted as the expiration time itself.

CALLOUT_FLAG_ROUNDUP
    Roundup the expiration time to the next resolution boundary. If this
    flag is not specified, the expiration time is rounded down.

References:
    https://www.illumos.org/issues/3582
    illumos/illumos-gate@0689f76

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #304

config/spl-build.m4

@@ -93,6 +93,7 @@ AC_DEFUN([SPL_AC_CONFIG_KERNEL], [
 	SPL_AC_RWSEM_SPINLOCK_IS_RAW
 	SPL_AC_SCHED_RT_HEADER
 	SPL_AC_2ARGS_VFS_GETATTR
+	SPL_AC_USLEEP_RANGE
 ])
 
 AC_DEFUN([SPL_AC_MODULE_SYMVERS], [
@@ -2400,3 +2401,25 @@ AC_DEFUN([SPL_AC_2ARGS_VFS_GETATTR], [
 		])
 	])
 ])
+
+dnl #
+dnl # 2.6.36 API compatibility.
+dnl # Added usleep_range timer.
+dnl # usleep_range is a finer precision implementation of msleep
+dnl # designed to be a drop-in replacement for udelay where a precise
+dnl # sleep / busy-wait is unnecessary.
+dnl #
+AC_DEFUN([SPL_AC_USLEEP_RANGE], [
+	AC_MSG_CHECKING([whether usleep_range() is available])
+	SPL_LINUX_TRY_COMPILE([
+		#include <linux/delay.h>
+	],[
+		usleep_range(0, 0);
+	],[
+		AC_MSG_RESULT(yes)
+		AC_DEFINE(HAVE_USLEEP_RANGE, 1,
+		          [usleep_range is available])
+	],[
+		AC_MSG_RESULT(no)
+	])
+])