lockd: Move share.h from include/linux/lockd/ to fs/lockd/

lockd: Move share.h from include/linux/lockd/ to fs/lockd/

The share.h header defines struct nlm_share and declares the DOS
share management functions used by the NLM server to implement
NLM_SHARE and NLM_UNSHARE operations. These interfaces are used
exclusively within the lockd subsystem. A git grep search confirms
no external code references them.

Relocating this header from include/linux/lockd/ to fs/lockd/
narrows the public API surface of the lockd module. Out-of-tree
code cannot depend on these internal interfaces after this change.
Future refactoring of the share management implementation thus
requires no consideration of external consumers.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>