xfs: introduce xfs_inode_buf.c for inode buffer operations

xfs: introduce xfs_inode_buf.c for inode buffer operations

The only thing remaining in xfs_inode.[ch] are the operations that
read, write or verify physical inodes in their underlying buffers.
Move all this code to xfs_inode_buf.[ch] and so we can stop sharing
xfs_inode.[ch] with userspace.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Review-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>