introduce __sb_writers_{acquired,release}() helpers

Preparation to hide the sb->s_writers internals from xfs and btrfs.
Add 2 trivial define's they can use rather than play with ->s_writers
directly. No changes in btrfs/transaction.o and xfs/xfs_aops.o.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Jan Kara <jack@suse.com>
diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
index f5021fc..a8ab8f5 100644
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -1638,9 +1638,7 @@
 	 * Tell lockdep about it.
 	 */
 	if (ac->newtrans->type & __TRANS_FREEZABLE)
-		rwsem_acquire_read(
-		     &ac->root->fs_info->sb->s_writers.lock_map[SB_FREEZE_FS-1],
-		     0, 1, _THIS_IP_);
+		__sb_writers_acquired(ac->root->fs_info->sb, SB_FREEZE_FS);
 
 	current->journal_info = ac->newtrans;
 
@@ -1679,9 +1677,7 @@
 	 * async commit thread will be the one to unlock it.
 	 */
 	if (ac->newtrans->type & __TRANS_FREEZABLE)
-		rwsem_release(
-			&root->fs_info->sb->s_writers.lock_map[SB_FREEZE_FS-1],
-			1, _THIS_IP_);
+		__sb_writers_release(root->fs_info->sb, SB_FREEZE_FS);
 
 	schedule_work(&ac->work);
 
diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
index 3859f5e..9bbb350 100644
--- a/fs/xfs/xfs_aops.c
+++ b/fs/xfs/xfs_aops.c
@@ -119,8 +119,7 @@
 	 * We may pass freeze protection with a transaction.  So tell lockdep
 	 * we released it.
 	 */
-	rwsem_release(&ioend->io_inode->i_sb->s_writers.lock_map[SB_FREEZE_FS-1],
-		      1, _THIS_IP_);
+	__sb_writers_release(ioend->io_inode->i_sb, SB_FREEZE_FS);
 	/*
 	 * We hand off the transaction to the completion thread now, so
 	 * clear the flag here.
@@ -171,8 +170,7 @@
 	 * Similarly for freeze protection.
 	 */
 	current_set_flags_nested(&tp->t_pflags, PF_FSTRANS);
-	rwsem_acquire_read(&VFS_I(ip)->i_sb->s_writers.lock_map[SB_FREEZE_FS-1],
-			   0, 1, _THIS_IP_);
+	__sb_writers_acquired(VFS_I(ip)->i_sb, SB_FREEZE_FS);
 
 	return xfs_setfilesize(ip, tp, ioend->io_offset, ioend->io_size);
 }