No system call to determine MAX_NUMNODES?

[Ralph Campbell <rcampbell@nvidia.com>]

I was using the latest git://git.cmpxchg.org/linux-mmotm.git and noticed 
a new issue compared to 5.0.0-rc5.

It looks like there is no convenient way to query the kernel's value for 
MAX_NUMNODES yet this is used in kernel_get_mempolicy() to validate the 
'maxnode' parameter to the GET_MEMPOLICY(2) system call.
Otherwise, EINVAL is returned.

Searching the internet for get_mempolicy yields some references that 
recommend reading /proc/<pid>/status and parsing the line "Mems_allowed:".

Running "cat /proc/self/status | grep Mems_allowed:" I get:
With 5.0.0-rc5:
Mems_allowed:   00000000,00000001
With 5.0.0-rc5-mm1:
Mems_allowed:   1
(both kernels were config'ed with CONFIG_NODES_SHIFT=6)

Clearly, there should be a better way to query MAX_NUMNODES like 
sysconf(), sysctl(), or libnuma.

I searched for the patch that changed /proc/self/status but didn't find it.

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