[RFC] delayed allocation for ext4

[Alex Tomas <alex@clusterfs.com>]

Good day,

probably the previous set of patches (including mballoc/lg)
is too large. so, I reworked delayed allocation a bit so
that it can be used on top of regular balloc, though it
still can be used with extents-enabled files only.

this time series contains just 3 patches:

 - booked-page-flag.patch
   adds PG_booked bit to page->flags. it's used in delayed
   allocation to mark space is already reserved for page
   (including possible metadata)

 - ext4-block-reservation.patch
   this is scalable free space management. every time we
   delay allocation of some page, a space (including metadata)
   should be reserved

 - ext4-delayed-allocation.patch
   delayed allocation itself, enabled by "delalloc" mount option.
   extents support is also required. currently it works only
   with blocksize=pagesize.


all the patches can be used in ftp://ftp.clusterfs.com/pub/people/alex/2.6.20-rc1/

the series passed basic tests like dd/dbench/fsx.

any comments/questions are very welcome.

thanks, Alex