cifs: don't overwrite dentry name in d_revalidate

Instead, use fatfs's method for dealing with negative dentries to
preserve case, rather than overwrite dentry name in d_revalidate, which
is a bit ugly and also gets in the way of doing lock-free path walking.

Signed-off-by: Nick Piggin <npiggin@kernel.dk>
Signed-off-by: Steve French <sfrench@us.ibm.com>