common/populate: Ensure that S_IFDIR.FMT_BTREE is in btree format

common/populate: Ensure that S_IFDIR.FMT_BTREE is in btree format

Sometimes "$((128 * dblksz / 40))" dirents cannot make sure that
S_IFDIR.FMT_BTREE could become btree format for its DATA fork.

Actually we just observed it can fail after apply our inode
extent-to-btree workaround. The root cause is that the kernel may be
too good at allocating consecutive blocks so that the data fork is
still in extents format.

Therefore instead of using a fixed number, let's make sure the number
of extents is large enough than (inode size - inode core size) /
sizeof(xfs_bmbt_rec_t).

Reviewed-by: Zorro Lang <zlang@redhat.com>
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>
Suggested-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Signed-off-by: Ziyang Zhang <ZiyangZhang@linux.alibaba.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>