linux-user: fix assertion in shmdt

shmdt fails to call mmap_lock/mmap_unlock around page_set_flags,
resulting in the following assertion:
  page_set_flags: Assertion `have_mmap_lock()' failed.

Wrap shmdt internals into mmap_lock/mmap_unlock.

Cc: qemu-stable@nongnu.org
Cc: Riku Voipio <riku.voipio@iki.fi>
Cc: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20180228221609.11265-7-jcmvbkbc@gmail.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>