If a file mmaped or pointed by exe link is unlinked, we will
generate a ghost file for it. On restore the ghost file will
be created with the users counter 1 and the very first open
(e.g. for mmap) will unlink the file.

Handle this by bumping up user counter for every mapping
pointing on the file.

This appeared after previous patches that packed the reg-files
image. Before it each vma and exe link created separate entry
in the reg-files image.
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>