mm/fadvise: discard partial page if endbyte is also EOF

During our recent testing with fadvise(FADV_DONTNEED), we find that if
given offset/length is not page-aligned, the last page will not be
discarded.  The tool we use is vmtouch (https://hoytech.com/vmtouch/),
we map a 10KB-sized file into memory and then try to run this tool to
evict the whole file mapping, but the last single page always remains
staying in the memory:

$./vmtouch -e test_10K
           Files: 1
     Directories: 0
   Evicted Pages: 3 (12K)
         Elapsed: 2.1e-05 seconds

$./vmtouch test_10K
           Files: 1
     Directories: 0
  Resident Pages: 1/3  4K/12K  33.3%
         Elapsed: 5.5e-05 seconds

However when we test with an older kernel, say 3.10, this problem is
gone.  So we wonder if this is a regression:

$./vmtouch -e test_10K
           Files: 1
     Directories: 0
   Evicted Pages: 3 (12K)
         Elapsed: 8.2e-05 seconds

$./vmtouch test_10K
           Files: 1
     Directories: 0
  Resident Pages: 0/3  0/12K  0%  <-...