fs/xfs: add missed symbol readlink_copy

When compile davinci_all_defconfig in Kevin's kernelci.org,
The missed symbol experting cause following error:

  ERROR: "readlink_copy" [fs/xfs/xfs.ko] undefined!

This patch fixed the problem.

Signed-off-by: Alex Shi <alex.shi@linaro.org>
diff --git a/fs/namei.c b/fs/namei.c
index 0fbf150..e3b23a0 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -4340,6 +4340,7 @@
 out:
 	return len;
 }
+EXPORT_SYMBOL(readlink_copy);
 
 /*
  * A helper for ->readlink().  This should be used *ONLY* for symlinks that