vfs_streams_xattr: stream names may contain colons

vfs_streams_xattr: stream names may contain colons

With vfs_fruit option "fruit:encoding = native" we're already converting
stream names that contain illegal NTFS characters from their on-the-wire
Unicode Private Range encoding to their native ASCII representation.

As as result the name of xattrs storing the streams (via
vfs_streams_xattr) may contain a colon, so we have to use strrchr_m()
instead of strchr_m() for matching the stream type suffix.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=11278

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>