So we don't keep holding it throughout the expensive the
fork/clone/send cycle of figuring out a namespace's init
pid.
Also change some functions to pass the init namespace inode
instead of the process' struct stat, so that it is obvious
that we don't use other parts of the stat info. The reason
is that we previously acquired the store lock before doing
the call to `stat()` and therefore it may not be immediately
obvious that things such as the timestamps found inside the
`struct stat` are not contributing to any race between the
multiple acquisitions of the store lock.
Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>