squashfs: fix buffer release race condition in readahead code

Fix a buffer release race condition, where the error value was used after
release.

Link: https://lkml.kernel.org/r/[e-mail]
Fixes: b09a7a036d20 ("squashfs: support reading fragments in readahead call")
Signed-off-by: Phillip Lougher <[e-mail]>
Tested-by: Bagas Sanjaya <[e-mail]>
Reported-by: Marc Miltenberger <[e-mail]>
Cc: Dimitri John Ledkov <[e-mail]>
Cc: Hsin-Yi Wang <[e-mail]>
Cc: Mirsad Goran Todorovac <[e-mail]>
Cc: Slade Watkins <[e-mail]>
Cc: Thorsten Leemhuis <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

fs/squashfs/file.c

···
 		squashfs_i(inode)->fragment_size);
 	struct squashfs_sb_info *msblk = inode->i_sb->s_fs_info;
 	unsigned int n, mask = (1 << (msblk->block_log - PAGE_SHIFT)) - 1;
+	int error = buffer->error;
 
-	if (buffer->error)
+	if (error)
 		goto out;
 
 	expected += squashfs_i(inode)->fragment_offset;
···
 
 out:
 	squashfs_cache_put(buffer);
-	return buffer->error;
+	return error;
 }
 
 static void squashfs_readahead(struct readahead_control *ractl)