ext4: the _require checks for bigalloc need a larger fs size

ext4: the _require checks for bigalloc need a larger fs size

_require_ext4_bigalloc and _require_ext4_mkfs_bigalloc create a
200k fs. DAX (as XIP before it) relies on a fs block size ==
system page size; ie 4k for us.  With a 200k fs size limit,
mkfs refuses to create a filesystem with a 4k block size, so
the check fails, but for a bogus reason.
Increasing the fs size to 512m allows mkfs to create a filesystem
with 4k block size, so the test passes.

Signed-off-by: Vishal Verma <vishal.l.verma@linux.intel.com>
Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>