xen: check if there are clients waiting in gnttab_end_foreign_access_references

Without a call to check_free_callbacks() clients waiting for grant
references would not be woken up even when there are sufficient grant
references available.

The check was likely left out as a mistake when the function was first
added.

Note that other functions used to free grant references already call
check_free_callbacks.

Submitted by: pratyush
Reviewed by: royger
Differential review: https://reviews.freebsd.org/D15899