percpu: fix synchronization between synchronous map extension and chunk destruction

For non-atomic allocations, pcpu_alloc() can try to extend the area
map synchronously after dropping pcpu_lock; however, the extension
wasn't synchronized against chunk destruction and the chunk might get
freed while extension is in progress.

This patch fixes the bug by putting most of non-atomic allocations
under pcpu_alloc_mutex to synchronize against pcpu_balance_work which
is responsible for async chunk management including destruction.

Signed-off-by: Tejun Heo <[email protected]>
Reported-and-tested-by: Alexei Starovoitov <[email protected]>
Reported-by: Vlastimil Babka <[email protected]>
Reported-by: Sasha Levin <[email protected]>
Cc: [email protected] # v3.18+
Fixes: 1a4d76076cda ("percpu: implement asynchronous chunk population")

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