exec: Transform exec_update_mutex into a rw_semaphore

Recently syzbot reported[0] that there is a deadlock amongst the users
of exec_update_mutex.  The problematic lock ordering found by lockdep
was:

   perf_event_open  (exec_update_mutex -> ovl_i_mutex)
   chown            (ovl_i_mutex       -> sb_writes)
   sendfile         (sb_writes         -> p->lock)
     by reading from a proc file and writing to overlayfs
   proc_pid_syscall (p->lock           -> exec_update_mutex)

While looking at possible solutions it occured to me that all of the
users and possible users involved only wanted to state of the given
process to remain the same.  They are all readers.  The only writer is
exec.

There is no reason for readers to block on each other.  So fix
this deadlock by transforming exec_update_mutex into a rw_semaphore
named exec_update_lock that only exec takes for writing.

Cc: Jann Horn <[email protected]>
Cc: Vasiliy Kulikov <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Bernd Edlinger <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Christopher Yeoh <[email protected]>
Cc: Cyrill Gorcunov <[email protected]>
Cc: Sargun Dhillon <[email protected]>
Cc: Christian Brauner <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Arnaldo Carvalho de Melo <[email protected]>
Fixes: eea9673250db ("exec: Add exec_update_mutex to replace cred_guard_mutex")
[0] https://lkml.kernel.org/r/[email protected]
Reported-by: [email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Eric W. Biederman <[email protected]>

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