file_ra_state_init() assumes that the file_ra_state has been zeroed out. 
Fixes a KMSAN used-unintialized issue (at least).

Fixes: cf948cbc ("cramfs: read_mapping_page() is synchronous")
Reported-by: syzbot <syzbot+8ce7f8308d91e6b8bbe2@syzkaller.appspotmail.com>
  Link: https://lkml.kernel.org/r/0000000000008f74e905f56df987@google.com


Cc: Matthew Wilcox <willy@infradead.org>
Cc: Nicolas Pitre <nico@fluxnic.net>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>