[JFFS2] fix write deadlock regression

I've bisected the deadlock when many small appends are done on jffs2 down to
this commit:

commit 6fe6900e
Author: Nick Piggin <npiggin@suse.de>
Date:   Sun May 6 14:49:04 2007 -0700

    mm: make read_cache_page synchronous

    Ensure pages are uptodate after returning from read_cache_page, which allows
    us to cut out most of the filesystem-internal PageUptodate calls.

    I didn't have a great look down the call chains, but this appears to fixes 7
    possible use-before uptodate in hfs, 2 in hfsplus, 1 in jfs, a few in
    ecryptfs, 1 in jffs2, and a possible cleared data overwritten with readpage in
    block2mtd.  All depending on whether the filler is async and/or can return
    with a !uptodate page.

It introduced a wait to read_cache_page, as well as a
read_cache_page_async function equivalent to the old read_cache_page
without any callers.

Switching jffs2_gc_fetch_page to read_cache_page_async for the old
behavior makes the deadlocks go away, but maybe reintroduces the
use-before-uptodate problem? I don't understand the mm/fs interaction
well enough to say.

[It's fine. dwmw2.]
Signed-off-by: NJason Lunz <lunz@falooley.org>
Signed-off-by: NDavid Woodhouse <dwmw2@infradead.org>

fs/jffs2/fs.c

@@ -627,7 +627,7 @@ unsigned char *jffs2_gc_fetch_page(struct jffs2_sb_info *c,
 	struct inode *inode = OFNI_EDONI_2SFFJ(f);
 	struct page *pg;
 
-	pg = read_cache_page(inode->i_mapping, offset >> PAGE_CACHE_SHIFT,
+	pg = read_cache_page_async(inode->i_mapping, offset >> PAGE_CACHE_SHIFT,
 			     (void *)jffs2_do_readpage_unlock, inode);
 	if (IS_ERR(pg))
 		return (void *)pg;