Instead of returning NULL for all errors, distinguish between:

 - no entry found and not asked to allocated (-ENOENT)
 - failed to allocate memory (-ENOMEM)
 - would block (-EAGAIN)

so that callers don't have to guess the error based on the passed in
flags.

Also pass through the error through the direct callers: filemap_get_folio,
filemap_lock_folio filemap_grab_folio and filemap_get_incore_folio.

[hch@lst.de: fix null-pointer deref]
  Link: https://lkml.kernel.org/r/20230310070023.GA13563@lst.de
  Link: https://lkml.kernel.org/r/20230310043137.GA1624890@u2004
Link: https://lkml.kernel.org/r/20230307143410.28031-8-hch@lst.deSigned-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Ryusuke Konishi <konishi.ryusuke@gmail.com> [nilfs2]
Cc: Andreas Gruenbacher <agruenba@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Naoya Horiguchi <naoya.horiguchi@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>