Use min_t(size_t, ...) instead of plain min(), which does strict type
checking, to avoid compile warning on i386.

Cc: Jianpeng Ma <majianpeng@gmail.com>
Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>