There are several places where we test if 'mid' is really the area NNNN in
the diagram and the tests have two variants and are non-obvious to follow.
Instead, set 'mid' to NULL up-front if it's not the NNNN area, and
simplify the tests.

Also update the description in comment accordingly.

[vbabka@suse.cz: adjust/add comments as suggested by Lorenzo]
  Link: https://lkml.kernel.org/r/def43190-53f7-a607-d1b0-b657565f4288@suse.cz
Link: https://lkml.kernel.org/r/20230309111258.24079-7-vbabka@suse.czSigned-off-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Lorenzo Stoakes <lstoakes@gmail.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>