simplify checks for I_CLEAR/I_FREEING

add I_CLEAR instead of replacing I_FREEING with it.  I_CLEAR is
equivalent to I_FREEING for almost all code looking at either;
it's there to keep track of having called clear_inode() exactly
once per inode lifetime, at some point after having set I_FREEING.
I_CLEAR and I_FREEING never get set at the same time with the
current code, so we can switch to setting i_flags to I_FREEING | I_CLEAR
instead of I_CLEAR without loss of information.  As the result of
such change, checks become simpler and the amount of code that needs
to know about I_CLEAR shrinks a lot.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

include/linux/fs.h

@@ -1616,8 +1616,8 @@ struct super_operations {
  * I_FREEING		Set when inode is about to be freed but still has dirty
  *			pages or buffers attached or the inode itself is still
  *			dirty.
- * I_CLEAR		Set by clear_inode().  In this state the inode is clean
- *			and can be destroyed.
+ * I_CLEAR		Added by clear_inode().  In this state the inode is clean
+ *			and can be destroyed.  Inode keeps I_FREEING.
  *
  *			Inodes that are I_WILL_FREE, I_FREEING or I_CLEAR are
  *			prohibited for many purposes.  iget() must wait for