[REGRESSION][BISECTED] 3.12-rc "n_tty: Don't wait for buffer work in read() loop" patch breaks gcc's testsuite

[Mikael Pettersson]

With 3.12-rc[12] I see unexpected failures in gcc's Ada acats testsuite, e.g.

                === acats tests ===
FAIL:   a83009b
FAIL:   c37209a
FAIL:   c45531e
FAIL:   c45614a
FAIL:   c67005d
FAIL:   c730a01
FAIL:   c74302b
FAIL:   cc3004a
FAIL:   cd2a24j
FAIL:   cd2a53a
FAIL:   cxa3001
FAIL:   cxf3a07
FAIL:   cxf3a08

                === acats Summary ===
# of expected passes            2307
# of unexpected failures        13
Native configuration is x86_64-unknown-linux-gnu

The exact failures vary from run to run, but some failures always occur on my
x86_64 machines, and all three open gcc branches (trunk, 4.8, 4.7) are affected.
With a 3.11 kernel the acats testsuite is always clean.

A bisection identified: