Now we can assume a kernel with recvmmsg support, this patch
simplifies the implementation to be similar to that for accept4:
either using socketcall or the syscall according to whether the
syscall is known to be available, without further fallback
implementations.
(In fact further simplification is possible, getting rid of the
__ASSUME_*_SYSCALL_WITH_SOCKETCALL macros now that the minimum kernel
is guaranteed support for all of accept4, recvmmsg, sendmmsg, whether
through syscalls or through socketcall. I intend to do that for all
of accept4 / recvmmsg / sendmmsg together - so making their
implementations just like those for older socket functions - once the
basic cleanup for 3.2 minimum kernel is done for sendmmsg as well as
recvmmsg.)
Tested for x86_64 and x86.
* sysdeps/unix/sysv/linux/kernel-features.h
(__ASSUME_RECVMMSG_SYSCALL): Define unconditionally.
(__ASSUME_RECVMMSG_SOCKETCALL): Remove macro.
(__ASSUME_RECVMMSG): Likewise.
* sysdeps/unix/sysv/linux/recvmmsg.c (recvmmsg): Define using
recvmmsg syscall if it can be assumed to be present, socketcall
otherwise, with no fallback for runtime failure.
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