The call to vn_start_write_refed() from vfs_write_resume() with
'mplocked' set to 'true' exactly boils down to doing an increment of
'mnt_writeopcount', albeit with lots of unnecessary verifications.

Replace it with an inline incrementation. As the original call was the
last with 'mplocked' with 'true', remove the 'mplocked' parameter from
vfs_write_resume(), simplifying its code accordingly ('mplocked' always
false).

While here, in vfs_write_resume(), initialize 'error' out of the mount
lock.

MFC after: 1 week
Sponsored by: The FreeBSD Foundation