xfs: set firstfsb to NULLFSBLOCK before feeding it to _bmapi_write

xfs: set firstfsb to NULLFSBLOCK before feeding it to _bmapi_write

Source kernel commit: 4c1a67bd3606540b9b42caff34a1d5cd94b1cf65

We must initialize the firstfsb parameter to _bmapi_write so that it
doesn't incorrectly treat stack garbage as a restriction on which AGs
it can search for free space.

Fixes-coverity-id: 1402025
Fixes-coverity-id: 1415167
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>