Btrfs: subpagesize-blocksize: Get rid of whole page writes.

This commit brings back functions that set/clear EXTENT_WRITEBACK bits. These
are required to reliably clear PG_writeback page flag.

Signed-off-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>
Btrfs: subpagesize-blocksize: __btrfs_buffered_write: Reserve/release extents aligned to block size.

Currently, the code reserves/releases extents in multiples of PAGE_CACHE_SIZE
units. Fix this.

Signed-off-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>
Btrfs: subpagesize-blocksize: Define extent_buffer_head.

In order to handle multiple extent buffers per page, first we need to create a
way to handle all the extent buffers that are attached to a page.

This patch creates a new data structure 'struct extent_buffer_head', and moves
fields that are common to all extent buffers in a page from 'struct extent
buffer' to 'struct extent_buffer_head'

Also, this patch moves EXTENT_BUFFER_TREE_REF, EXTENT_BUFFER_DUMMY and
EXTENT_BUFFER_IN_TREE flags from extent_buffer->ebflags  to
extent_buffer_head->bflags.

Signed-off-by: Chandra Seetharaman <sekharan@us.ibm.com>
Signed-off-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>
Btrfs: subpagesize-blocksize: Read tree blocks whose size is <PAGE_CACHE_SIZE.

In the case of subpagesize-blocksize, this patch makes it possible to read
only a single metadata block from the disk instead of all the metadata blocks
that map into a page.

Signed-off-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>
Btrfs: subpagesize-blocksize: Write only dirty extent buffers belonging to a page

For the subpagesize-blocksize scenario, This patch adds the ability to write a
single extent buffer to the disk.

Signed-off-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>
Btrfs: subpagesize-blocksize: Allow mounting filesystems where sectorsize != PAGE_SIZE

This patch allows mounting filesystems with blocksize smaller than the
PAGE_SIZE.

Signed-off-by: Chandra Seetharaman <sekharan@us.ibm.com>
Signed-off-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>
Btrfs: subpagesize-blocksize: Compute and look up csums based on sectorsized blocks.

Checksums are applicable to sectorsize units. The current code uses
bio->bv_len units to compute and look up checksums. This works on machines
where sectorsize == PAGE_CACHE_SIZE. This patch makes the checksum
computation and look up code to work with sectorsize units.

Signed-off-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>