xfs/015: enlarge the initial fs size

xfs/015: enlarge the initial fs size

Kernel commit f59cf5c29919 ("xfs: remove "no-allocation"
reservations for file creations") is known to cause earlier ENOSPC
conditions, and xfs/015 is affected in the way that no new file/dir
can be created in a newly created 16M XFS with both reflink and
rmapbt enabled, thus xfs/015 fails due to the missing test dirs.

So enlarge the initial fs size to 32M that we're able to create new
dir/file before growing the filesystem size.

Note that we doubled the fs initial size, we need to double the
required space too.

Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>