mm/mmap.c: use helper function allow_write_access() in __remove_shared_vm_struct()

In commit 1da177e4 ("Linux-2.6.12-rc2"), the helper allow_write_access
came with the atomic_inc operation of the i_writecount field in the func
__remove_shared_vm_struct().  But it forgot to use this helper function.

Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Link: https://lkml.kernel.org/r/20200921115814.39680-1-linmiaohe@huawei.com


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>