tests: TestFileHoleCopy: accept +-4kB

Failure looked like this:

--- FAIL: TestFileHoleCopy (3.73s)
    --- FAIL: TestFileHoleCopy/k81 (0.04s)
        file_holes_test.go:93: size changed: st0.Blocks=88 st2.Blocks=96
    file_holes_test.go:147: aborting further subtests

$ findholes TestFileHoleCopy.k81.1
         0 data
     36864 hole
     45056 data
     50434 hole
     50434 eof

$ findholes TestFileHoleCopy.k81.2
         0 data
     36864 hole
     45056 data
     50434 hole
     50434 eof

$ filefrag -v TestFileHoleCopy.k81.1
Filesystem type is: ef53
File size of TestFileHoleCopy.k81.1 is 50434 (13 blocks of 4096 bytes)
 ext:     logical_offset:        physical_offset: length:   expected: flags:
   0:        0..       2:   23702311..  23702313:      3:
   1:        3..       8:   20389855..  20389860:      6:   23702314:
   2:       11..      12:   23702314..  23702315:      2:   20389863: last,eof
TestFileHoleCopy.k81.1: 3 extents found

$ filefrag -v TestFileHoleCopy.k81.2
Filesystem type is: ef53
File size of TestFileHoleCopy.k81.2 is 50434 (13 blocks of 4096 bytes)
 ext:     logical_offset:        physical_offset: length:   expected: flags:
   0:        0..       2:   20389861..  20389863:      3:
   1:        3..       4:   23702316..  23702317:      2:   20389864:
   2:        5..       6:   20389864..  20389865:      2:   23702318:
   3:        7..       8:   23702318..  23702319:      2:   20389866:
   4:       11..      12:   23702320..  23702321:      2:             last,eof
TestFileHoleCopy.k81.2: 4 extents found

0 files changed, 0 insertions, 0 deletions