x86/extable: switch to using FIELD_GET_SIGNED()

x86/extable: switch to using FIELD_GET_SIGNED()

The EX_DATA register is laid out such that EX_DATA_IMM occupied MSB.
It's done to make sure that FIELD_GET() will sign-extend the IMM
field during extraction.

To enforce that, all EX_DATA masks are made signed integers. This
works, but relies on the particular implementation of FIELD_GET(),
i.e. masking then shifting, not vice versa; and the particular
placement of the fields in the register.

Switch to using the dedicated FIELD_GET_SIGNED(), and relax those
limitations.

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Yury Norov <ynorov@nvidia.com>