mm: Initialize error in shmem_file_aio_read()

Some versions of gcc even warn about it:

  mm/shmem.c: In function ‘shmem_file_aio_read’:
  mm/shmem.c:1414: warning: ‘error’ may be used uninitialized in this function

If the loop is aborted during the first iteration by one of the two
first break statements, error will be uninitialized.

Introduced by commit 6e58e79d ("introduce copy_page_to_iter, kill
loop over iovec in generic_file_aio_read()").

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>