fs: remove mpage_alloc

open code mpage_alloc in it's two callers and simplify the results
because of the context:

 - __mpage_writepage always passes GFP_NOFS and can thus always sleep and
    will never get a NULL return from bio_alloc at all.
 - do_mpage_readpage can only get a non-sleeping context for readahead
   which never sets PF_MEMALLOC and thus doesn't need the retry loop
   either.

Both cases will never have __GFP_HIGH set.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220124091107.642561-2-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>