src/rbd.cc: silence CID COPY_PASTE_ERROR warning

CID 1021212 (#1 of 1): Copy-paste error (COPY_PASTE_ERROR)
  copy_paste_error: "r" in "r = -*__errno_location()" looks like
  a copy-paste error. Should it say "fd" instead?

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>