exofs: Move exofs specific osd operations out of ios.c

ios.c will be moving to an external library, for use by the
objects-layout-driver. Remove from it some exofs specific functions.

Also g_attr_logical_length is used both by inode.c and ios.c
move definition to the later, to keep it independent

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