mm: replace hardcoded 3% with admin_reserve_pages knob

Add an admin_reserve_kbytes knob to allow admins to change the hardcoded
memory reserve to something other than 3%, which may be multiple
gigabytes on large memory systems.  Only about 8MB is necessary to
enable recovery in the default mode, and only a few hundred MB are
required even when overcommit is disabled.

This affects OVERCOMMIT_GUESS and OVERCOMMIT_NEVER.

admin_reserve_kbytes is initialized to min(3% free pages, 8MB)

I arrived at 8MB by summing the RSS of sshd or login, bash, and top.

Please see first patch in this series for full background, motivation,
testing, and full changelog.

[[email protected]: coding-style fixes]
[[email protected]: make init_admin_reserve() static]
Signed-off-by: Andrew Shewmaker <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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