UFS snapshots: properly set the vm object size.

UFS snapshots: properly set the vm object size.

Citing Kirk:
The previous code [before 8563de2f2799b2cb -- kib] did not call
vnode_pager_setsize() but worked because later in ffs_snapshot() it
does a UFS_WRITE() to output the snaplist. Previously the UFS_WRITE()
allocated the extra block at the end of the file which caused it to do
the needed vnode_pager_setsize(). But the new code had already allocated
the extra block, so UFS_WRITE() did not extend the size and thus did not
do the vnode_pager_setsize().

PR:	253158
Reported by:	Harald Schmalzbauer <bugzilla.freebsd@omnilan.de>
Reviewed by:	mckusick
Tested by:	cy
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week