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>
diff --git a/mm/filemap.c b/mm/filemap.c
index 847ee43c..30ab120 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2684,11 +2684,11 @@
 	struct inode *inode = file->f_mapping->host;
 	ssize_t ret;
 
-	mutex_lock(&inode->i_mutex);
+	inode_lock(inode);
 	ret = generic_write_checks(iocb, from);
 	if (ret > 0)
 		ret = __generic_file_write_iter(iocb, from);
-	mutex_unlock(&inode->i_mutex);
+	inode_unlock(inode);
 
 	if (ret > 0) {
 		ssize_t err;