Remove redunand i_dev and i_fs pointers, which are available as
ip->i_ump->um_dev and ip->i_ump->um_fs, and reorder members by size to
reduce padding.  To compensate added derefences, the most often i_ump
access to differentiate between UFS1 and UFS2 dinode layout is
removed, by addition of the new i_flag IN_UFS2.  Overall, this
actually reduces the amount of memory dereferences.

On 64bit machine, original struct inode size is 176, reduced to 152
bytes with the change.

Tested by:	pho (previous version)
Reviewed by:	mckusick
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks