xfs: fix iclog allocation size

Properly allocate the space for the bio_vecs instead of just one byte
per bio_vec.

Fixes: 79b54d9b ("xfs: use bios directly to write log buffers")
Reported-by:  <syzbot+b75afdbe271a0d7ac4f6@syzkaller.appspotmail.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>