newfs_msdos: align data area to VM page boundary by default

Without alignment, the data area will not be aligned with the buffer
cache, leading to overhead, higher write multiplication on SSD devices
and issues with very large cluster sizes (see PR 277414).

The -A option used to align the start of the root directory to a
multiple of the cluster size, which happens to align the start of the
data area with a buffer page boundary in case of large clusters and
the default number of directory entries (512 entries requiring 16 KB
for FAT12 or FAT16, FAT32 puts the root directory into the data area).

This commit aligns the start of the data area with the page size, if
neither -A nor -r is used. It changes -A to align the start of the
data area (end of the root directory) to a multiple of the cluster
size, since this is the alignment that prevents write multiplication
due to clusters crossing erase block boundaries of a SSD device.
The -r option is unchanged and will prevent any automatic alignment
from occuring.

Approved by: kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D45436