9p: set readahead and io size according to maxsize

having a readahead of 128k with a msize of 128k (with overhead) lead to
reading 124+4k everytime, making two roundtrips needlessly.

tune readahead according to msize when cache is enabled for better
performance

Link: http://lkml.kernel.org/r/20211104120323.2189376-1-asmadeus@codewreck.orgSigned-off-by: Dominique Martinet <asmadeus@codewreck.org>

fs/9p/vfs_super.c

@@ -79,6 +79,9 @@ v9fs_fill_super(struct super_block *sb, struct v9fs_session_info *v9ses,
 	if (!v9ses->cache) {
 		sb->s_bdi->ra_pages = 0;
 		sb->s_bdi->io_pages = 0;
+	} else {
+		sb->s_bdi->ra_pages = v9ses->maxdata >> PAGE_SHIFT;
+		sb->s_bdi->io_pages = v9ses->maxdata >> PAGE_SHIFT;
 	}
 
 	sb->s_flags |= SB_ACTIVE | SB_DIRSYNC;