Add a usecs_to_jiffies() function.  This will be used in one of my
subsequent patches.  With the potential for dynamic HZ values much higher
than 1000, we may need to consider times as small as usecs in terms of
jiffies.  We have msecs_to_jiffies(), jiffies_to_msecs() and
jiffies_to_usecs(), but no usecs_to_jiffies().
Signed-off-by: Nishanth Aravamudan <nacc@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>