perf: Break deadlock involving exec_update_mutex

Syzbot reported a lock inversion involving perf. The sore point being
perf holding exec_update_mutex() for a very long time, specifically
across a whole bunch of filesystem ops in pmu::event_init() (uprobes)
and anon_inode_getfile().

This then inverts against procfs code trying to take
exec_update_mutex.

Move the permission checks later, such that we need to hold the mutex
over less code.

Reported-by:  <syzbot+db9cdf3dd1f64252c6ef@syzkaller.appspotmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>