misc/mk_hugefiles: simplify get_partition_start()

misc/mk_hugefiles: simplify get_partition_start()

search_sysfs_block() is causing -Wformat-truncation warnings.  These
could be fixed by checking the return value of snprintf(), instead of
doing buggy checks like 'strlen(p_de->d_name) > SYSFS_PATH_LEN -
strlen(path) - 32', which has an integer underflow bug.

However, the only purpose of search_sysfs_block() is to find the sysfs
directory for a block device by device number.  That can trivially be
done using /sys/dev/block/$major:$minor.  So just do that instead.  Also
make get_partition_start() explicitly Linux-only, as it has never worked
anywhere else.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>