mips: Do not malloc on getdents64 fallback

This patch changes how the fallback getdents64 implementation calls
non-LFS getdents by replacing the scratch_buffer with static buffer
plus a loop on getdents calls.  This avoids the potential malloc
call on scratch_buffer_set_array_size for large input buffer size
at the cost of more getdents syscalls.

It also adds a small optimization for older kernels, where the first
ENOSYS failure for getdents64 disable subsequent calls.

Check the dirent tests on a mips64-linux-gnu with getdents64 code
disabled.

	* sysdeps/unix/sysv/linux/mips/mips64/getdents64.c (__getdents64):
	Add small optimization for older kernel to avoid issuing
	__NR_getdents64 on each call and replace scratch_buffer usage with
	a static allocated buffer.

Reviewed-by: Florian Weimer <fweimer@redhat.com>