xfs: fix uninitialized field in rtbitmap fsmap backend

Initialize the extent count field of the high key so that when we use
the high key to synthesize an 'unknown owner' record (i.e. used space
record) at the end of the queried range we have a field with which to
compute rm_blockcount.  This is not strictly necessary because the
synthesizer never uses the rm_blockcount field, but we can shut up the
static code analysis anyway.

Coverity-id: 1437358
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>