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In CL 6350, Brad fixed the following system calls
to use the program-wide workding directory:

- bind
- chdir
- create
- open
- remove
- stat
- umount
- wstat

However, Russ Cox pointed out that the mount
system call should be fixed as well.

Change-Id: I6139ed11ba449f18c46e95269f4d0e51be7cec48
Reviewed-on: https://go-review.googlesource.com/6385

Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>