xfs_repair: don't crash on ENOSPC rebuilding a btree

xfs_repair: don't crash on ENOSPC rebuilding a btree

During btree rebuilding, the cursor setup function checks ext_ptr to
report ENOSPC problems when it grabs the first extent for the btree.
However, subsequent grabs for free space don't check ext_ptr and so we
segfault if there's no space.  Therefore, move the ENOSPC check into
the loop so that we always complain about insufficient space instead
of just crashing.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>