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>

fs/squashfs/symlink.c

@@ -119,7 +119,7 @@ const struct address_space_operations squashfs_symlink_aops = {
 
 const struct inode_operations squashfs_symlink_inode_ops = {
 	.readlink = generic_readlink,
-	.follow_link = page_follow_link_light,
+	.get_link = page_get_link,
 	.put_link = page_put_link,
 	.getxattr = generic_getxattr,
 	.listxattr = squashfs_listxattr