output file cannot be in the UBIFS root directory check is not working quite well

[kyak]

Hi,

Consider the following sequence of commands:

cd ~
mkdir -p /tmp/wtf/wtf1
chmod u-r /tmp/wtf
mkfs.ubifs -m 4096 -e 516096 -c 4095 -d /tmp/wtf/wtf1 -o mytest.img
Error: output file cannot be in the UBIFS root directory

mytest.img is not inside /tmp/wtf/wtf1, but mkfs.ubifs fails because 
/tmp/wtf is not readable.
It seems that implementation of in_path(..) in mkfs.ubifs.c is not quite 
correct to handle this type of situations.

If you are wondering, my /home is not readable, and this prevents me from 
building any ubifs image inside of /home/user.

Thanks in advance.