Staging: binder: mmap fixes.

Only allow a binder file pointer to be mmapped once. The buffer management
code cannot deal with more then one area.
Also remove leftover mutex_unlock if mmap fails.

Signed-off-by: Arve Hjønnevåg <arve@android.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>