Invalidate inode extents cache on truncation.

It is needed to invalidate cache in case of inode space removal
to avoid situation, when extents cache returns not exist extent.

Reviewed by:    pfg
MFC after:      2 weeks
Differential Revision:  https://reviews.freebsd.org/D29931

sys/fs/ext2fs/ext2_extents.c

@@ -1623,5 +1623,6 @@ ext4_ext_remove_space(struct inode *ip, off_t length, int flags,
 	ext4_ext_drop_refs(path);
 	free(path, M_EXT2EXTENTS);
 
+	ip->i_ext_cache.ec_type = EXT4_EXT_CACHE_NO;
 	return (error);
 }