xtensa: fix fast_syscall_spill_registers

The original implementation could clobber registers under certain conditions.

The Xtensa processor architecture uses windowed registers and the original
implementation was using a4 as a temporary register, which under certain
conditions could be register a0 of the oldest window frame, and didn't always
restore the content correctly.

By moving the _spill_registers routine inside the fast system call, it frees
up one more register (the return address is not required anymore) for the
spill routine.
Signed-off-by: Chris Zankel <chris@zankel.net>

arch/xtensa/kernel/vectors.S

@@ -235,7 +235,7 @@ ENTRY(_DoubleExceptionVector)
 
 	/* Check for overflow/underflow exception, jump if overflow. */
 
-	_bbci.l	a0, 6, _DoubleExceptionVector_WindowOverflow
+	bbci.l	a0, 6, _DoubleExceptionVector_WindowOverflow
 
 	/*
 	 * Restart window underflow exception.

arch/xtensa/kernel/xtensa_ksyms.c

@@ -122,9 +122,7 @@ EXPORT_SYMBOL(insw);
 EXPORT_SYMBOL(insl);
 
 extern long common_exception_return;
-extern long _spill_registers;
 EXPORT_SYMBOL(common_exception_return);
-EXPORT_SYMBOL(_spill_registers);
 
 #ifdef CONFIG_FUNCTION_TRACER
 EXPORT_SYMBOL(_mcount);