tracing/user_events: Split up mm alloc and attach

When a new mm is being created in a fork() path it currently is
allocated and then attached in one go. This leaves the mm exposed out to
the tracing register callbacks while any parent enabler locations are
copied in. This should not happen.

Split up mm alloc and attach as unique operations. When duplicating
enablers, first alloc, then duplicate, and only upon success, attach.
This prevents any timing window outside of the event_reg mutex for
enablement walking. This allows for dropping RCU requirement for
enablement walking in later patches.

Link: https://lkml.kernel.org/r/[email protected]
Link: https://lore.kernel.org/linux-trace-kernel/CAHk-=whTBvXJuoi_kACo3qi5WZUmRrhyA-_=rRFsycTytmB6qw@mail.gmail.com/

Signed-off-by: Linus Torvalds <[email protected]>
[ change log written by Beau Belgrave ]
Signed-off-by: Beau Belgrave <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>

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