namei: allow set_root() to produce errors

For LOOKUP_BENEATH and LOOKUP_IN_ROOT it is necessary to ensure that
set_root() is never called, and thus (for hardening purposes) it should
return an error rather than permit a breakout from the root. In
addition, move all of the repetitive set_root() calls to nd_jump_root().

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