Impact: code cleanup

This patch rename pa_linear to pa_type and add MB_INODE_PA
and MB_GROUP_PA to indicate inode and group prealloc space.
Signed-off-by: NAneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Reviewed-by: NEric Sandeen <sandeen@redhat.com>
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>