mm/hugetlb: remove VM_BUG_ON(!nrg) in get_file_region_entry_from_cache()

mm/hugetlb: remove VM_BUG_ON(!nrg) in get_file_region_entry_from_cache()

We are sure to get a valid file_region, otherwise the
VM_BUG_ON(resv->region_cache_count <= 0) at the very beginning would be
triggered.

Let's remove the redundant one.

Signed-off-by: Wei Yang <richard.weiyang@linux.alibaba.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Link: https://lkml.kernel.org/r/20200831022351.20916-3-richard.weiyang@linux.alibaba.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>