iomap: handle error conditions more gracefully in iomap_to_bh

iomap_to_bh currently BUG()s when the passed in block number is not
in the iomap.  For file systems that have proper synchronization this
should never happen and so far hasn't in mainline, but for block devices
size changes aren't fully synchronized against ongoing I/O.  Instead
of BUG()ing in this case, return -EIO to the caller, which already has
proper error handling.  While we're at it, also return -EIO for an
unknown iomap state instead of returning garbage.

Fixes: 487c607d ("block: use iomap for writes to block devices")
Reported-by:  <syzbot+4a08ffdf3667b36650a1@syzkaller.appspotmail.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>