dtrace: fix resolving addresses of relocation records for SDT probe points

dtrace: fix resolving addresses of relocation records for SDT probe points

The addresses were being calculated based on the wrong starting point (_stext
whereas it ought to be _text), and the base was not taken into account.
Fixed the writing of NOPs in the location of the probe point calls, since
the existing case was causing kernel paging faults.  Made the add_nops()
function in alternative.c non-static so it can be used in sdt_register.
Use add_nops() to select the most appropriate NOP sequence for replacing the
probe point call, and write the NOPs using text_poke().

Signed-off-by: Kris Van Hees <kris.van.hees@oracle.com>