Use is_afpXXX_stream() functions
Don't overwrite creation time if was previously set by make_create_timespec

smb2_create calls dos_mode() after calling SMB_VFS_CREATE_FILE(), as a
result the creation time as set by make_create_timespec() is always
overwritten.

Add a check to dos_mode() that tests for st_ex_calculated_birthtime ==
true and only then update the creation time with the value from the
DOS attribute xattr.

Signed-off-by: Ralph Boehme <rb@sernet.de>