Speeding up df

[Philipp Thomas]

To help a customer that complained about the time it takes for df to check
one of their machines with nearly 22000 lines in /proc/self/mountinfo, my
collegue came up with this patch that uses a hash table for seaching in 
filter_mount_list() and get_dev(). Data from said machine:

Without patch:
real    0m2.273s
user    0m0.768s
sys     0m1.500s

real    0m1.731s
user    0m0.532s
sys     0m1.188s

With patch:
real    0m1.286s
user    0m0.052s
sys     0m1.232s

real    0m1.066s
user    0m0.028s
sys     0m1.032s

Is this patch acceptable?

Philipp