xfs: fix rounding in xfs_free_file_space

The offset passed into xfs_free_file_space() needs to be rounded
down to a certain size, but the rounding mask is built by a 32 bit
variable. Hence the mask will always mask off the upper 32 bits of
the offset and lead to incorrect writeback and invalidation ranges.

This is not actually exposed as a bug because we writeback and
invalidate from the rounded offset to the end of the file, and hence
the offset we are actually punching a hole out of will always be
covered by the code. This needs fixing, however, if we ever want to
use exact ranges for writeback/invalidation here...

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Ben Myers <bpm@sgi.com>

(cherry picked from commit 28ca489c)