sflist: SYS_SFLIST_FLAGS_MASK must be a long not an int

When splitting the pointer from the flag, ~SYS_SFLIST_FLAGS_MASK remains
a 32-bit value because of the lack of an L qualifier. Let's qualify it
with UL so the top half of 64-bit pointers is not truncated.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>