don't put symlink bodies in pagecache into highmem

kmap() in page_follow_link_light() needed to go - allowing to hold
an arbitrary number of kmaps for long is a great way to deadlocking
the system.

new helper (inode_nohighmem(inode)) needs to be used for pagecache
symlinks inodes; done for all in-tree cases.  page_follow_link_light()
instrumented to yell about anything missed.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/udf/namei.c b/fs/udf/namei.c
index d0e6de1..42eafb9 100644
--- a/fs/udf/namei.c
+++ b/fs/udf/namei.c
@@ -922,6 +922,7 @@
 
 	inode->i_data.a_ops = &udf_symlink_aops;
 	inode->i_op = &page_symlink_inode_operations;
+	inode_nohighmem(inode);
 
 	if (iinfo->i_alloc_type != ICBTAG_FLAG_AD_IN_ICB) {
 		struct kernel_lb_addr eloc;