mke2fs: account for physical as well as logical sector size

mke2fs: account for physical as well as logical sector size

Some devices, notably 4k sector drives, may have a 512 logical
sector size, mapped onto a 4k physical sector size.

When mke2fs is ratcheting down the blocksize for small filesystems,
or when a blocksize is specified on the commandline, we should not
willingly go below the physical sector size of the device.

When a blocksize is specified, we -must- not go below
the logical sector size of the device.

Add a new library function, ext2fs_get_device_phys_sectsize()
to get the physical sector size if possible, and adjust the
logic in mke2fs to enforce the above rules.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>