linux-user: Do not treat madvise()'s advice as a bitmask

Advice is enum, not flags. Doing (advice & MADV_DONTNEED) also matches
e.g. MADV_MERGEABLE.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20220725134100.128035-1-iii@linux.ibm.com>
Fixes: 892a4f6a ("linux-user: Add partial support for MADV_DONTNEED")
Signed-off-by: Laurent Vivier <laurent@vivier.eu>