locks: revise generic_add_lease tracepoint

Now that check_conflicting_open uses inode->i_readcount instead of
the dentry->d_count to detect opens for read, revise the tracepoint
to display that value instead.

Also, fl is never NULL, so no need to check for that in the fast
assign section.

Cc: Amir Goldstein <amir73il@gmail.com>
Reported-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Jeff Layton <jlayton@kernel.org>