VFS: we need to set LOOKUP_JUMPED on mountpoint crossing

Mountpoint crossing is similar to following procfs symlinks - we do
not get ->d_revalidate() called for dentry we have arrived at, with
unpleasant consequences for NFS4.

Simple way to reproduce the problem in mainline:

cat >/tmp/a.c <<'EOF'
#include <unistd.h>
#include <fcntl.h>
#include <stdio.h>
main()
{
struct flock fl = {.l_type = F_RDLCK, .l_whence = SEEK_SET, .l_len = 1};
if (fcntl(0, F_SETLK, &fl))
perror("setlk");
}
EOF
cc /tmp/a.c -o /tmp/test

then on nfs4:

mount --bind file1 file2
/tmp/test < file1 # ok
/tmp/test < file2 # spews "setlk: No locks available"...

What happens is the missing call of ->d_revalidate() after mountpoint
crossing and that's where NFS4 would issue OPEN request to server.

The fix is simple - treat mountpoint crossing the same way we deal with
following procfs-style symlinks. I.e. set LOOKUP_JUMPED...

Cc: stable@kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>