exportfs: remove locking around ->get_parent() call.

The locking around the ->get_parent() call brings no value.
We are locking a child which is only used to find an inode and thence the
parent inode number.  All further activity involves the parent inode
which may have several children so locking one child cannot protect the
parent in any useful way.

The filesystem must already ensure that only one 'struct inode' exists
for a given inode, and will call d_obtain_alias() which contains the
required locking to ensure only one dentry will be attached to that
inode.

So remove the unnecessary locking.

Signed-off-by: NeilBrown <neil@brown.name>
Link: https://lore.kernel.org/r/174190497326.9342.9313518146512158587@noble.neil.brown.name


Signed-off-by: Christian Brauner <brauner@kernel.org>