The page allocation bitmaps in the extent descriptor pages
contain two bits per page: XDES_FREE_BIT and XDES_CLEAN_BIT,
which is unused. Simplify read access.

xdes_is_free(descr,mtr): Remove. Use !xdes_get_n_used(descr) instead.

xdes_is_free(): Replaces xdes_get_bit(), xdes_mtr_get_bit().

xdes_find_free(): Replaces xdes_find_bit().

fsp_seg_inode_page_get_nth_inode(): Remove the redundant parameters
physical_size, mtr.

fsp_seg_inode_page_find_used(), fsp_seg_inode_page_find_free():
Remove the redundant parameter mtr.