VFS: repack LOOKUP_ bit flags.

VFS: repack LOOKUP_ bit flags.

The LOOKUP_ bits are not in order, which can make it awkward when adding
new bits.  Two bits have recently been added to the end which makes them
look like "scoping flags", but in fact they aren't.

Also LOOKUP_PARENT is described as "internal use only" but is used in
fs/nfs/

This patch:
 - Moves these three flags into the "pathwalk mode" section
 - changes all bits to use the BIT(n) macro
 - Allocates bits in order leaving gaps between the sections,
   and documents those gaps.

Signed-off-by: NeilBrown <neilb@suse.de>
Link: https://lore.kernel.org/r/20250206054504.2950516-8-neilb@suse.de
Signed-off-by: Christian Brauner <brauner@kernel.org>