arch/microblaze: support get_user() of size 8 bytes

arch/microblaze: support get_user() of size 8 bytes

arch/microblaze/ is missing support for get_user() of size 8 bytes,
so add it by using __copy_from_user().

While there, also drop a lot of the code duplication.

Fixes these build errors:
   drivers/infiniband/core/uverbs_main.o: In function `ib_uverbs_write':
   drivers/infiniband/core/.tmp_gl_uverbs_main.o:(.text+0x13a4): undefined reference to `__user_bad'
   drivers/android/binder.o: In function `binder_thread_write':
   drivers/android/.tmp_gl_binder.o:(.text+0xda6c): undefined reference to `__user_bad'
   drivers/android/.tmp_gl_binder.o:(.text+0xda98): undefined reference to `__user_bad'
   drivers/android/.tmp_gl_binder.o:(.text+0xdf10): undefined reference to `__user_bad'
   drivers/android/.tmp_gl_binder.o:(.text+0xe498): undefined reference to `__user_bad'
   drivers/android/binder.o:drivers/android/.tmp_gl_binder.o:(.text+0xea78): more undefined references to `__user_bad' follow

'make allmodconfig' now builds successfully for arch/microblaze/.

Fixes: 538722ca3b76 ("microblaze: fix get_user/put_user side-effects")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Steven J. Magnani <steve@digidescorp.com>
Cc: Michal Simek <monstr@monstr.eu>
Cc: Jason Gunthorpe <jgg@mellanox.com>
Cc: Leon Romanovsky <leonro@mellanox.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Doug Ledford <dledford@redhat.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>