fsnotify: dump tmpfs notify watchers by names (v2)

Currently file handles are used for dumping {i,fs}notify watchers.

But inode numbers are not restored for tmpfs content, so watchers can't
be opened by handles.

Pavel found, that tmpfs cache is not pruned, so a handle can be opened,
and readlink(/proc/PID/fd/X) will return a corect path to the file.

v2: use read_fd_link()
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>