mmu: fix vma locking during mmap related operations

Right now most of mmap related functions have the same bug related to vma
locking: they validate mapping under vma lock then release the lock
and do actual vma operation, but since between validation and operation
the mapping can go away it is incorrect. This patch fixes this by doing
validation and operation under the same lock.

Signed-off-by: Gleb Natapov <gleb@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>