vfs: fix MNT_SYNCHRONOUS check in vn_write

ca1ce50b2b5ef11d ("vfs: add more safety against concurrent forced
unmount to vn_write") has a side effect of only checking MNT_SYNCHRONOUS
if O_FSYNC is set.

Reviewed By: mjg
Differential Revision: https://reviews.freebsd.org/D30610

(cherry picked from commit a19ae1b099ad4d43588f15ef19b8506f606b27cb)