mgr/nfs: reuse CephfsClient for path checks and earmark resolver

mgr/nfs: reuse CephfsClient for path checks and earmark resolver

cephfs_path_is_dir defined an inner function decorated with lru_cache, so
each call got a new function object and an empty cache, CephfsClient(mgr)
ran every time. Moved caching to module-level cephfs_client_for_mgr(mgr)
and call it from cephfs_path_is_dir.
Passed that shared client into CephFSEarmarkResolver from the NFS module so
export create/apply does not construct a separate CephfsClient for
earmarks.

Fixes: https://tracker.ceph.com/issues/76504
Signed-off-by: Shweta Bhosale <Shweta.Bhosale1@ibm.com>