kill-the-BKL/reiserfs: release the write lock inside reiserfs_read_bitmap_block()

reiserfs_read_bitmap_block() uses sb_bread() to read the bitmap block. This
helper might sleep.

Then, when the bkl was used, it was released at this point. We can then
relax the write lock too here.

[ Impact: release the reiserfs write lock when it is not needed ]

Cc: Jeff Mahoney <jeffm@suse.com>
Cc: Chris Mason <chris.mason@oracle.com>
Cc: Alexander Beregalov <a.beregalov@gmail.com>
Signed-off-by: NFrederic Weisbecker <fweisbec@gmail.com>

fs/reiserfs/bitmap.c

@@ -1249,7 +1249,9 @@ struct buffer_head *reiserfs_read_bitmap_block(struct super_block *sb,
 	else if (bitmap == 0)
 		block = (REISERFS_DISK_OFFSET_IN_BYTES >> sb->s_blocksize_bits) + 1;
 
+	reiserfs_write_unlock(sb);
 	bh = sb_bread(sb, block);
+	reiserfs_write_lock(sb);
 	if (bh == NULL)
 		reiserfs_warning(sb, "sh-2029: %s: bitmap block (#%u) "
 		                 "reading failed", __func__, block);