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.

Fixes: a3ab7155ea21 ("omfs: add directory routines")
Signed-off-by: Hyungjung Joo <jhj140711@gmail.com>
Link: https://patch.msgid.link/20260317054827.1822061-1-jhj140711@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>