wrappers for ->i_mutex access

parallel to mutex_{lock,unlock,trylock,is_locked,lock_nested},
inode_foo(inode) being mutex_foo(&inode->i_mutex).

Please, use those for access to ->i_mutex; over the coming cycle
->i_mutex will become rwsem, with ->lookup() done with it held
only shared.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/xfs/xfs_pnfs.c

@@ -42,11 +42,11 @@ xfs_break_layouts(
 	while ((error = break_layout(inode, false) == -EWOULDBLOCK)) {
 		xfs_iunlock(ip, *iolock);
 		if (with_imutex && (*iolock & XFS_IOLOCK_EXCL))
-			mutex_unlock(&inode->i_mutex);
+			inode_unlock(inode);
 		error = break_layout(inode, true);
 		*iolock = XFS_IOLOCK_EXCL;
 		if (with_imutex)
-			mutex_lock(&inode->i_mutex);
+			inode_lock(inode);
 		xfs_ilock(ip, *iolock);
 	}