Support symlinks in kernels that expect .readlink

Starting with kernel version 4.10, a NULL value for the ->readlink()
method defaults to generic_readlink(), or to a similar codepath.  For
older kernels this field must be set explicitly, otherwise the readlink
syscall will fail.

Signed-off-by: Ernesto A. Fernández <ernesto.mnd.fernandez@gmail.com>

symlink.c

@@ -64,4 +64,7 @@ const struct inode_operations apfs_symlink_inode_operations = {
 	.get_link	= apfs_get_link,
 	.getattr	= apfs_getattr,
 	.listxattr	= apfs_listxattr,
+#if LINUX_VERSION_CODE < KERNEL_VERSION(4, 10, 0) /* Now this is the default */
+	.readlink	= generic_readlink,
+#endif
 };