Auto merge of #750 - agrover:fix-devt-minor, r=alexcrichton
Auto merge of #750 - agrover:fix-devt-minor, r=alexcrichton

Fix dev_t minor() bitmasking on Linux

This code appears to be modeled on the macros in glibc bits/sysmacros.h
(since Glibc 2.26). Fix the masking of bits for minor() to match that
implementation, which also corresponds with the explanatory comment in
that file.

cactorium/libc,rust-lang/libc,rust-lang/libc,cactorium/libc,rust-lang/libc,rust-lang-nursery/libc,rust-lang-nursery/libc,cactorium/libc,rust-lang-nursery/libc