LU-1436 debug: return the original type in RETURN

in RETURN, the bit length of rc could be 32 or 64, and it also could
be pointer type, the conversion betweens pointer and integer type
will trigger compile warning related to type casting, then define an
static variable of typeof(rc), and avoid the stack usage ATM.

Signed-off-by: Hongchao Zhang <hongchao.zhang@whamcloud.com>
Change-Id: I7f3a54438dfe1f294168691718c4db8116f68de4
Reviewed-on: http://review.whamcloud.com/3072
Tested-by: Hudson
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>