Fix a "v_seqc_users == 0 not met" panic when VFS_STATFS() fails during mount.

r363210 introduced v_seqc_users to the vnodes. This change requires
a vn_seqc_write_end() to match the vn_seqc_write_begin() in
vfs_cache_root_clear().
mjg@ provided this patch which seems to fix the panic.

Tested for an NFS mount where the VFS_STATFS() call will fail.

Submitted by: mjg
Reviewed by: mjg
Differential Revision: https://reviews.freebsd.org/D26160