uprobes: Shift ->readpage check from __copy_insn() to uprobe_register()

copy_insn() fails with -EIO if ->readpage == NULL, but this error
is not propagated unless uprobe_register() path finds ->mm which
already mmaps this file. In this case (say) "perf record" does not
actually install the probe, but the user can't know about this.

Move this check into uprobe_register() so that this problem can be
detected earlier and reported to user.

Note: this is still not perfect,

	- copy_insn() and arch_uprobe_analyze_insn() should be called
	  by uprobe_register() but this is not simple, we need vm_file
	  for read_mapping_page() (although perhaps we can pass NULL),
	  and we need ->mm for is_64bit_mm() (although this logic is
	  broken anyway).

	- uprobe_register() should be called by create_trace_uprobe(),
	  not by probe_event_enable(), so that an error can be detected
	  at "perf probe -x" time. This also needs more changes in the
	  core uprobe code, uprobe register/unregister interface was
	  poorly designed from the very beginning.

Reported-by: Denys Vlasenko <[email protected]>
Signed-off-by: Oleg Nesterov <[email protected]>
Acked-by: Srikar Dronamraju <[email protected]>
Signed-off-by: Peter Zijlstra <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Arnaldo Carvalho de Melo <[email protected]>
Cc: Linus Torvalds <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

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