reiserfs: remove IS_PRIVATE helpers

There are a number of helper functions for marking a reiserfs inode
private that were leftover from reiserfs did its own thing wrt to
private inodes.  S_PRIVATE has been in the kernel for some time, so this
patch removes the helpers and uses IS_PRIVATE instead.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>