[RFC] efivars write(2) races

[Al Viro]

	1) process A does write() on efivars file, reaches ->get_variable(),
gets newdatasize set, drops efivars->lock and loses CPU before an attempt to
grab ->i_mutex.  process B comes and does the same thing, replacing the
variable contents.  Then it grabs ->i_mutex, updates size, drops ->i_mutex
and buggers off.  At which point A gets CPU back and proceeds to set size
to whatever would be valid for its write.  Only the value is bogus now...

	2) what's to prevent EFI_NOT_FOUND being hit twice?  Bad things
will obviously happen in that case...

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