libceph: use pg_num_mask instead of pgp_num_mask for pg.seed calc

commit 9542cf0bf9b1a3adcc2ef271edbcbdba03abf345 upstream.

Fix a typo that used the wrong bitmask for the pg.seed calculation.  This
is normally unnoticed because in most cases pg_num == pgp_num.  It is, however,
a bug that is easily corrected.

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Alex Elder <alex.elder@linary.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>