Change default bufsize to 512k

Change default bufsize to 512k

I did not do any scientific tests, but the dummy test on my laptop against a
w2k3 vmware shows that in this specific situation going beyond 512k seems not
to gain anything anymore.

smb: \> iosize 64512
iosize is now 64512
smb: \> lcd /dev
smb: \> get random null
getting file \random of size 104857600 as null (33453,1 kb/s) (average 19718,5 kb/s)
smb: \> get random null
getting file \random of size 104857600 as null (34236,0 kb/s) (average 20509,1 kb/s)
smb: \> iosize 524288
iosize is now 524288
smb: \> get random null
getting file \random of size 104857600 as null (49042,1 kb/s) (average 22521,9 kb/s)
smb: \> get random null
getting file \random of size 104857600 as null (49420,8 kb/s) (average 23432,9 kb/s)
smb: \> iosize 1048576
iosize is now 1048576
smb: \> get random null
getting file \random of size 104857600 as null (49420,8 kb/s) (average 24284,2 kb/s)
smb: \> get random null
getting file \random of size 104857600 as null (49136,3 kb/s) (average 25076,9 kb/s)
(This used to be commit 1bcfef6a9093aa873550a263dc142c8a3f5edffd)