replace ->follow_link() with new method that could stay in RCU mode

new method: ->get_link(); replacement of ->follow_link().  The differences
are:
	* inode and dentry are passed separately
	* might be called both in RCU and non-RCU mode;
the former is indicated by passing it a NULL dentry.
	* when called that way it isn't allowed to block
and should return ERR_PTR(-ECHILD) if it needs to be called
in non-RCU mode.

It's a flagday change - the old method is gone, all in-tree instances
converted.  Conversion isn't hard; said that, so far very few instances
do not immediately bail out when called in RCU mode.  That'll change
in the next commits.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/coda/cnode.c b/fs/coda/cnode.c
index dd6a79e..f18139c 100644
--- a/fs/coda/cnode.c
+++ b/fs/coda/cnode.c
@@ -18,7 +18,7 @@
 
 static const struct inode_operations coda_symlink_inode_operations = {
 	.readlink	= generic_readlink,
-	.follow_link	= page_follow_link_light,
+	.get_link	= page_get_link,
 	.put_link	= page_put_link,
 	.setattr	= coda_setattr,
 };