namei: d_is_negative() should be checked before ->d_seq validation

Fetching ->d_inode, verifying ->d_seq and finding d_is_negative() to
be true does *not* mean that inode we'd fetched had been NULL - that
holds only while ->d_seq is still unchanged.

Shift d_is_negative() checks into lookup_fast() prior to ->d_seq
verification.

Reported-by: Steven Rostedt <rostedt@goodmis.org>
Tested-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>