git: HAMMER VFS - Disallow rebalancing on small-memory machines

[Thomas Nikolajsen]

>commit f6468e9eb2c9de9b9af113b10cd1a0b3c64c0e42
>Author: Matthew Dillon <dillon at apollo.backplane.com>
>Date:   Tue Jan 26 12:50:33 2010 -0800
>
>    HAMMER VFS - Disallow rebalancing on small-memory machines
>    
>    * Rebalancing may have to hold upwards of 3900 buffers locked
>      in the worst case, disallow the operation on machines which
>      do not configure enough buffer cache buffers.

Could this be MFC to 2.4, thanks.
Just cherry-picking works here.

ENOSPC isn't the most obvious error code:
when testing I first thought something wasn't right,
as 'No space left on device' isn't correct.
What about EDEADLK?
(could match the stall I had before this commit (issue1552))
It seems we don't have error message just saying 'resource shortage'.

 -thomas