zfs snapshot: tool path is non-verridable , one call bypasses ZFS_COMMAND,

[ThalesBarretto]

Summary

xlators/mgmt/glusterd/src/snapshot/glusterd-zfs-snapshot.c reaches the zfs
utility through a file-local macro:

#define ZFS_COMMAND "/sbin/zfs"

Two related problems:

1. One call bypasses the macro. Six of the seven zfs invocations in the file
   go through ZFS_COMMAND, but glusterd_zfs_dataset() spells the zfs list
   command as a bare "zfs" string (line 54).
2. Not overridable. ZFS_COMMAND bakes in /sbin/zfs with no way for a packager
   to point it elsewhere without patching the source — unlike the sibling path macros
   _PATH_UMOUNT and _PATH_SETFATTR, which are #ifndef-guarded for exactly this.

The outlier:

/* glusterd_zfs_dataset(), line 54 */
runner_add_args(&runner, "zfs", "list", "-Ho", "name", brick_path, NULL);

Note the two spellings resolve differently: glusterd's runner spawns via
posix_spawnp, so the bare "zfs" is $PATH-resolved, whereas ZFS_COMMAND pins
/sbin/zfs. So line 54 is not just cosmetically different — it is the one site whose
zfs path is not the file's declared /sbin/zfs.