powerpc/fscr: Enable interrupts earlier before calling get_user()

The function get_user() can sleep while trying to fetch instruction
from user address space and causes the following warning from the
scheduler.

BUG: sleeping function called from invalid context

Though interrupts get enabled back but it happens bit later after
get_user() is called. This change moves enabling these interrupts
earlier covering the function get_user(). While at this, lets check
for kernel mode and crash as this interrupt should not have been
triggered from the kernel context.

Signed-off-by: Anshuman Khandual <khandual@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>