fs/omfs: reject s_sys_blocksize smaller than OMFS_DIR_START

In the Linux kernel, the following vulnerability has been resolved:

fs/omfs: reject s_sys_blocksize smaller than OMFS_DIR_START

omfs_fill_super() rejects oversized s_sys_blocksize values (> PAGE_SIZE),

but it does not reject values smaller than OMFS_DIR_START (0x1b8 = 440).

Later, omfs_make_empty() uses

sbi->s_sys_blocksize - OMFS_DIR_START

as the length argument to memset(). Since s_sys_blocksize is u32,

a crafted filesystem image with s_sys_blocksize < OMFS_DIR_START causes

an unsigned underflow there, wrapping to a value near 2^32. That drives

a ~4 GiB memset() from bh->b_data + OMFS_DIR_START and overwrites kernel

memory far beyond the backing block buffer.

Add the corresponding lower-bound check alongside the existing upper-bound

check in omfs_fill_super(), so that malformed images are rejected during

superblock validation before any filesystem data is processed.