namei.c: separate the parts of follow_link() that find the link body

Split a piece of fs/namei.c:follow_link() that does obtaining the link
body into a separate function.  follow_link() itself is converted to
calling get_link() and then doing the body traversal (if any).

The next step will expand follow_link() call in link_path_walk()
and this helps to keep the size down...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>