I got this wrong in 69695c23 for the
case that the segment spans two pages.

Ref: #2541

mmaps from b25.vtc varnishstat:

before this change:

6598  open("/tmp/vtc.6492.2b3b5875/v1//_.vsm_child/_.VSC_cluster.322833fd50de0860", O_RDONLY) = 21
6598  mmap(NULL, 4096, PROT_READ, MAP_SHARED, 21, 0) = 0x7fb92529e000
...
6598  open("/tmp/vtc.6492.2b3b5875/v1//_.vsm_child/_.VSC_cluster.322833fd50de0860", O_RDONLY) = 21
6598  mmap(NULL, 8192, PROT_READ, MAP_SHARED, 21, 0) = 0x7fb925271000
...
6598  open("/tmp/vtc.6492.2b3b5875/v1//_.vsm_child/_.VSC_cluster.322833fd50de0860", O_RDONLY) = 21
6598  mmap(NULL, 12288, PROT_READ, MAP_SHARED, 21, 0x1000) = 0x7fb925222000
...
6598  open("/tmp/vtc.6492.2b3b5875/v1//_.vsm_child/_.VSC_cluster.322833fd50de0860", O_RDONLY) = 21
6598  mmap(NULL, 12288, PROT_READ, MAP_SHARED, 21, 0x2000) = 0x7fb92521f000

after this change:

7749  open("/tmp/vtc.7648.2c3869d9/v1//_.vsm_child/_.VSC_cluster.09720dc34dd3559c", O_RDONLY) = 21
7749  mmap(NULL, 4096, PROT_READ, MAP_SHARED, 21, 0) = 0x7f0938a28000
...
7749  open("/tmp/vtc.7648.2c3869d9/v1//_.vsm_child/_.VSC_cluster.09720dc34dd3559c", O_RDONLY) = 21
7749  mmap(NULL, 4096, PROT_READ, MAP_SHARED, 21, 0x1000) = 0x7f09389f9000
...
7749  open("/tmp/vtc.7648.2c3869d9/v1//_.vsm_child/_.VSC_cluster.09720dc34dd3559c", O_RDONLY) = 21
7749  mmap(NULL, 8192, PROT_READ, MAP_SHARED, 21, 0x1000) = 0x7f09389d2000
...
7749  open("/tmp/vtc.7648.2c3869d9/v1//_.vsm_child/_.VSC_cluster.09720dc34dd3559c", O_RDONLY) = 21
7749  mmap(NULL, 4096, PROT_READ, MAP_SHARED, 21, 0x2000) = 0x7f09389d1000

Notice that we keep the mappings at one or two pages now and do not increase
the length with the offset.