smb: client: preserve leading slash for POSIX absolute symlink targets

smb: client: preserve leading slash for POSIX absolute symlink targets

When creating a native SMB symbolic link (CIFS_SYMLINK_TYPE_NATIVE) whose
target is an absolute path on a mount that uses POSIX paths, the leading
path separator was silently dropped from the stored symlink target.

create_native_symlink() converted the target to UTF-16 with
cifs_convert_path_to_utf16().  That helper was intended for share-relative
SMB paths and therefore unconditionally strips a leading path separator.
For an absolute POSIX symlink target the leading '/' is significant, so a
target of "/foo/bar" was stored and read back as "foo/bar", even
though the reparse point was still flagged as absolute
(SYMLINK_FLAG_RELATIVE cleared).

On a POSIX paths mount the symlink target is stored verbatim, so convert
it directly with cifs_strndup_to_utf16() instead.  This preserves the
leading separator, avoids the leading-backslash stripping that
cifs_convert_path_to_utf16() also performs (a backslash is a valid POSIX
filename character), and uses NO_MAP_UNI_RSVD to match the readback path
in smb2_parse_native_symlink(), which always converts the target with
cifs_strndup_from_utf16() / NO_MAP_UNI_RSVD.  This mirrors how the NFS and
WSL reparse symlink creators convert their targets.

The NT-style absolute symlink handling, which needs the "\??\" prefix and
drive-letter colon preserved, continues to use cifs_convert_path_to_utf16()
together with the existing masking of those bytes.

Fixes: 12b466eb52d9 ("cifs: Fix creating and resolving absolute NT-style symlinks")
Reviewed-by: Paulo Alcantara (Red Hat) <pc@manguebit.org>
Acked-by: Ralph Boehme <slow@samba.org>
Signed-off-by: Steve French <stfrench@microsoft.com>