fsync02: SIGFPE fix

It may happen that stat_buf.f_frsize is greater than
BLOCKSIZE (for example, if we use "non-default" FS block sizes or NFS) and
because on (BLOCKSIZE / stat_buf.f_frsize) is 0.

Moved stat_buf.f_frsize to the dividend.

Signed-off-by: Stanislav Kholmanskikh <stanislav.kholmanskikh@oracle.com>