staging/lustre/llite: move root_squash from sysfs to debugfs

root_squash control got accidentally moved to sysfs instead of
debugfs, and the write side of it was also broken expecting a
userspace buffer.
It contains both uid and gid values in a single file, so debugfs
is a clear place for it.

Reported-by: Al Viro <viro@ZenIV.linux.org.uk>
Fixes: c948390f "fix inconsistencies of root squash feature"
Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>