mm/hugeltb: simplify the return code of __vma_reservation_common()

mm/hugeltb: simplify the return code of __vma_reservation_common()

It's guaranteed that the vma is associated with a resv_map, i.e.  either
VM_MAYSHARE or HPAGE_RESV_OWNER, when the code reaches here or we would
have returned via !resv check above.  So it's unneeded to check whether
HPAGE_RESV_OWNER is set here.  Simplify the return code to make it more
clear.

Link: https://lkml.kernel.org/r/20210410072348.20437-3-linmiaohe@huawei.com
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Cc: Feilong Lin <linfeilong@huawei.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>