cifs: Fix parsing native symlinks directory/file type

As SMB protocol distinguish between symlink to directory and symlink to
file, add some mechanism to disallow resolving incompatible types.

When SMB symlink is of the directory type, ensure that its target path ends
with slash. This forces Linux to not allow resolving such symlink to file.

And when SMB symlink is of the file type and its target path ends with
slash then returns an error as such symlink is unresolvable. Such symlink
always points to invalid location as file cannot end with slash.

As POSIX server does not distinguish between symlinks to file and symlink
directory, do not apply this change for symlinks from POSIX SMB server. For
POSIX SMB servers, this change does nothing.

This mimics Windows behavior of native SMB symlinks.

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>