generic/042: set size to 38M for f2fs

generic/042: set size to 38M for f2fs

25M is too small for f2fs:

Info: Disable heap-based policy
Info: Debug level = 0
Info: Label =
Info: Trim is enabled
Info: Segments per section = 1
Info: Sections per zone = 1
Info: sector size = 512
Info: total sectors = 51200 (25 MB)
Info: zone aligned segment0 blkaddr: 512
    Error: Device size is not sufficient for F2FS volume
    Error: Failed to prepare a super block!!!
    Error: Could not format the device!!!

So, set size to 38M for f2fs.

Signed-off-by: Sun Ke <sunke32@huawei.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>