libshare: use AVL tree with static data, pass all data in arguments

This makes it so we don't leak a consistent 64 bytes anymore,
makes the searches simpler and faster, removes /all allocations/
from the driver (quite trivially, since they were absolutely needless),
and makes libshare thread-safe (except, maybe, linux/smb, but that only
does pointer-width loads/stores so it's also mostly fine, except for
leaking smb_shares)

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #13165

lib/libshare/smb.h

@@ -43,5 +43,3 @@ typedef struct smb_share_s {
 
 	struct smb_share_s *next;
 } smb_share_t;
-
-void libshare_smb_init(void);