namei: take the treatment of absolute symlinks to get_link()

rather than letting the callers handle the jump-to-root part of
semantics, do it right in get_link() and return the rest of the
body for the caller to deal with - at that point it's treated
the same way as relative symlinks would be.  And return NULL
when there's no "rest of the body" - those are treated the same
as pure jump symlink would be.

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