gfs2: Do not call iomap_zero_range beyond eof

Since commit eb65540a ("iomap: warn on zero range of a post-eof
folio"), iomap_zero_range() warns when asked to zero a folio beyond eof.
The warning triggers on the following code path:

  gfs2_fallocate(FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE)
    __gfs2_punch_hole()
      gfs2_block_zero_range()
        iomap_zero_range()

In __gfs2_punch_hole(), gfs2 zeroes out partial folios at the beginning
and at the end of the specified range, whether those folios are beyond
eof or not.  This may add folios to the page cache which are entirely
beyond eof, which isn't of any use.  Avoid that by truncating the range
to zero out at eof.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>