hugetlb: fix error return for brk() entering a hugepage region

The lats commit causes the wrong return value.
is_hugepage_only_range() is a boolean, so we should return
-EINVAL rather than 1.

Also - we can use "mm" instead of looking up "current->mm" again.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Adrian Bunk <bunk@kernel.org>