filemap: Get vma mnt_id early

We have a, well, issue with how we calculate the vma's mnt_id.

Right now get one via criu side file descriptor that it got by
opening the /proc/pid/map_files/ link. The problem is that these
descriptors are 'merged' or 'borrowed' by adjacent vmas from
previous ones. Thus, getting the mnt_id value for each of them
makes no sense -- these files are the same.

So move this mnt_id getting earlier into vma parsing code. This
brings a potential problem -- if we have two adjacent vmas
mapping the same inode (dev:ino pair) but living in different
mount namespaces -- this check would produce wrong result.
"Wrong" from the perspective that on restore correct file would
be opened from wrong namespace.

I propose to live with it, since this is not worse than the
--evasive-devices option, it's _very_ unlikely, but saves a lot
of openeings.

Note, that in case app switched mount namespace and then mapped
some new library (with dlopen) things would work correctly -- new
vmas will likely be not adjacent and for different dev:ino.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

include/proc_parse.h

@@ -212,6 +212,7 @@ extern int parse_fdinfo_pid(int pid, int fd, int type,
 		int (*cb)(union fdinfo_entries *e, void *arg), void *arg);
 extern int parse_cpuinfo_features(int (*handler)(char *tok));
 extern int parse_file_locks(void);
+extern int get_fd_mntid(int fd, int *mnt_id);
 
 struct pid;
 extern int parse_threads(int pid, struct pid **_t, int *_n);