Detect kernels that honor gfp flags passed to vmalloc()

[ryao]

2092cf6 used PF_MEMALLOC
to workaround a bug in the Linux kernel where allocations did not honor
the gfp flags passed to vmalloc(). Unfortunately, PF_MEMALLOC has the
side effect of permitting allocations to allocate pages outside of
ZONE_NORMAL. This has been observed to result in the depletion of
ZONE_DMA32 on Gentoo Linux. A kernel patch is available in the Gentoo
bug tracker for this issue:

https://bugs.gentoo.org/show_bug.cgi?id=416685

This negates any benefit PF_MEMALLOC provides, so we introduce an
autotools check to disable the use of PF_MEMALLOC on systems with
patched kernels.

Signed-off-by: Richard Yao [email protected]

[behlendorf]

Merged with a few minor modifications to the autotools test to silence various gcc warnings. This isn't critical since -Werror isn't used here but it helps ensure the test succeeds or fails for exactly the right reasons, and that reason is clear.

• Prevent CONFIG_MMU from being redefined if it's already set
• Prevent RCH_HAS_4LEVEL_HACK from being undefined if it's NOT already set
• Silence uninit variable warnings by initializing pud, addr, gfp_mask before using them in an assignment. In the case of pud this was most easily done by simply changing it to a pud_t *