exofs: Move layout related members to a layout structure

* Abstract away those members in exofs_sb_info that are related/needed
  by a layout into a new exofs_layout structure. Embed it in exofs_sb_info.

* At exofs_io_state receive/keep a pointer to an exofs_layout. No need for
  an exofs_sb_info pointer, all we need is at exofs_layout.

* Change any usage of above exofs_sb_info members to their new name.

Signed-off-by: Boaz Harrosh <bharrosh@panasas.com>

diff --cc fs/exofs/exofs.h
Simple merge

diff --cc fs/exofs/inode.c
Simple merge

diff --cc fs/exofs/ios.c
Simple merge

diff --cc fs/exofs/super.c
Simple merge