Reset shmem_exit_inprogress after shmem_exit()

Reset shmem_exit_inprogress after shmem_exit()

In ad9a274778d2d88c46b90309212b92ee7fdf9afe, shmem_exit_inprogress was
introduced.  But we need to reset it after shmem_exit(), because unlike
the similar proc_exit(), shmem_exit() can also be called for cleanup
when the process will not exit.

Reported-by: Andrew Gierth <andrew@tao11.riddles.org.uk>