USB: g_file_storage: call allow_signal()

New changes in the signal-handling code require compensating changes
in g_file_storage.  This patch (as913) by Oleg Nesterov makes the
code use allow_signal() instead of sigprocmask().

From: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>