zfs_onexit_fd_hold: return EBADF even if devfs_get_cdevpriv gave ENOENT

/dev/zfs always has per-open data, so when it is missing the file
descriptor is for some other file.  Returning ENOENT in this case
is confusing as a variety of other conditions (like a missing dataset)
may result in the same error.  It's better to consistently return
EBADF for any problems with the file descriptor.

Note that zfs_onexit_fd_hold() is used with 'automatic cleanup fd'
- when that fd is closed, typically because a process is terminated,
some cleanup action is taken by ZFS driver.  E.g. a temporary
snapshot hold is released.

Perhaps, it would even be worthwhile changing devfs_get_cdevpriv()
to return EBADF if there is no associated data.

Differential Revision:	https://reviews.freebsd.org/D2370
Reviewed by:	delphij, smh
MFC after:	12 days

sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_onexit.c

@@ -137,7 +137,7 @@ zfs_onexit_fd_hold(int fd, minor_t *minorp)
 		*minorp = (minor_t)(uintptr_t)data;
 	curthread->td_fpop = tmpfp;
 	if (error != 0)
-		return (error);
+		return (SET_ERROR(EBADF));
 	return (zfs_onexit_minor_to_state(*minorp, &zo));
 }