Fix deadlock between zfs umount & snapentry_expire

Fix deadlock between zfs umount & snapentry_expire

zfs umount -> zfsctl_destroy() takes the zfs_snapshot_lock as a
writer and calls zfsctl_snapshot_unmount_cancel(), which waits
for snapentry_expire() if present (when snap is automounted).
This snapentry_expire() itself then waits for zfs_snapshot_lock
as a reader, resulting in a deadlock.

The fix is to only hold the zfs_snapshot_lock over the tree
lookup and removal.  After a successful lookup the lock can
be dropped and zfs_snapentry_t will remain valid until the
reference taken by the lookup is released.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rohan Puri <rohan.puri15@gmail.com>
Closes #7751
Closes #7752