mm/madvise: move up the behavior parameter validation

madvise_behavior_valid() should be called before acting upon the
behavior parameter.  Hence move up the function.  This also includes
MADV_SOFT_OFFLINE and MADV_HWPOISON options as valid behavior parameter
for the system call madvise().

Link: http://lkml.kernel.org/r/20170418052844.24891-1-khandual@linux.vnet.ibm.com
Signed-off-by: Anshuman Khandual <khandual@linux.vnet.ibm.com>
Acked-by: David Rientjes <rientjes@google.com>
Acked-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>