libceph: dump pg_temp mappings to debugfs

Dump pg_temp mappings to /sys/kernel/debug/ceph/<client>/osdmap,
one 'pg_temp <pgid> [<osd>, ..., <osd>]' per line, e.g:

    pg_temp 2.6 [2,3,4]

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Reviewed-by: Alex Elder <elder@linaro.org>