lib: introduce early boot parameter to avoid page_ext memory overhead

The highest memory overhead from memory allocation profiling comes from
page_ext objects.  This overhead exists even if the feature is disabled
but compiled-in.  To avoid it, introduce an early boot parameter that
prevents page_ext object creation.  The new boot parameter is a tri-state
with possible values of 0|1|never.  When it is set to "never" the memory
allocation profiling support is disabled, and overhead is minimized
(currently no page_ext objects are allocated, in the future more overhead
might be eliminated).  As a result we also lose ability to enable memory
allocation profiling at runtime (because there is no space to store
alloctag references).  Runtime sysctrl becomes read-only if the early boot
parameter was set to "never".  Note that the default value of this boot
parameter depends on the CONFIG_MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT
configuration.  When CONFIG_MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT=n the
boot parameter is set to "never", therefore eliminating any overhead. 
CONFIG_MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT=y results in boot parameter
being set to 1 (enabled).  This allows distributions to avoid any overhead
by setting CONFIG_MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT=n config and with
no changes to the kernel command line.

We reuse sysctl.vm.mem_profiling boot parameter name in order to avoid
introducing yet another control.  This change turns it into a tri-state
early boot parameter.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Suren Baghdasaryan <[email protected]>
Reviewed-by: Vlastimil Babka <[email protected]>
Tested-by: Kees Cook <[email protected]>
Cc: Alexander Viro <[email protected]>
Cc: Alex Gaynor <[email protected]>
Cc: Alice Ryhl <[email protected]>
Cc: Andreas Hindborg <[email protected]>
Cc: Benno Lossin <[email protected]>
Cc: "Björn Roy Baron" <[email protected]>
Cc: Boqun Feng <[email protected]>
Cc: Christoph Lameter <[email protected]>
Cc: Dennis Zhou <[email protected]>
Cc: Gary Guo <[email protected]>
Cc: Kent Overstreet <[email protected]>
Cc: Miguel Ojeda <[email protected]>
Cc: Pasha Tatashin <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: Wedson Almeida Filho <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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