vfs: handle vfs_init() failures

Most vfs_init implementations will not fail, with the notable current
exception that tmpfs_subr_init() can fail to allocate a new swap pager
type, in which case we probably do not want to proceed and keep it
registered. linsysfs was a potential consumer, but we opted to go a
different direction and move pseudofs init/deinit over to first mount
and last mount instead.

Reviewed by: fuz, kib
Differential Revision: https://reviews.freebsd.org/D52037