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>
(cherry picked from commit 3c5f6a5f888729f9fbc64211298f7c3e2fb42b64)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

linux-user/syscall.c

@@ -4918,6 +4918,9 @@ static inline abi_ulong do_shmat(CPUArchState *cpu_env,
 static inline abi_long do_shmdt(abi_ulong shmaddr)
 {
     int i;
+    abi_long rv;
+
+    mmap_lock();
 
     for (i = 0; i < N_SHM_REGIONS; ++i) {
         if (shm_regions[i].in_use && shm_regions[i].start == shmaddr) {
@@ -4926,8 +4929,11 @@ static inline abi_long do_shmdt(abi_ulong shmaddr)
             break;
         }
     }
+    rv = get_errno(shmdt(g2h(shmaddr)));
+
+    mmap_unlock();
 
-    return get_errno(shmdt(g2h(shmaddr)));
+    return rv;
 }
 
 #ifdef TARGET_NR_ipc