copy_xstate_to_kernel: Fix typo which caused GDB regression

This fixes a regression encountered while running the
gdb.base/corefile.exp test in GDB's test suite.

In my testing, the typo prevented the sw_reserved field of struct
fxregs_state from being output to the kernel XSAVES area.  Thus the
correct mask corresponding to XCR0 was not present in the core file for
GDB to interrogate, resulting in the following behavior:

   [kev@f32-1 gdb]$ ./gdb -q testsuite/outputs/gdb.base/corefile/corefile testsuite/outputs/gdb.base/corefile/corefile.core
   Reading symbols from testsuite/outputs/gdb.base/corefile/corefile...
   [New LWP 232880]

   warning: Unexpected size of section `.reg-xstate/232880' in core file.

With the typo fixed, the test works again as expected.

Signed-off-by: Kevin Buettner <kevinb@redhat.com>
Fixes: 9e463654 ("copy_xstate_to_kernel(): don't leave parts of destination uninitialized")
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Dave Airlie <airlied@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>