libext2fs: further clean up and rename check_block_uninit

libext2fs: further clean up and rename check_block_uninit

Commit 8e44eb64bb (libext2fs: mark group data blocks when loading
block bitmap) simplified check_block_uninit since we are now
initializing the bitmap when it is loaded from disk.  It left some
variables which were being set but never used, however.  In addition,
since we only need check_block_uninit() to clear the block bitmap's
uninit flag, rename it to clear_block_uninit(), and only call it once
we have found a free block in ext2fs_new_blocks2().

This cleans up the code some and optimizes things if we need to search
multiple block groups trying to find a free block.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: Darrick J. Wong <darrick.wong@oracle.com>