Update vn_set_pwd() to allow user|kernal address for filename

Update vn_set_pwd() to allow user|kernal address for filename

During module init spl_setup()->The vn_set_pwd("/") was failing
with -EFAULT because user_path_dir() and __user_walk() both
expect 'filename' to be a user space address and it's not in
this case.  To handle this the data segment size is increased
to to ensure strncpy_from_user() does not fail with -EFAULT.

Additionally, I've added a printk() warning to catch this and
log it to the console if it ever reoccurs.  I thought everything
was working properly here because there consequences of this
failing are subtle and usually non-critical.