ext4: fix return-value types in several function comments

ext4: fix return-value types in several function comments

The documentation comments for ext4_read_block_bitmap_nowait and
ext4_read_inode_bitmap describe them as returning NULL on error, but
they return an ERR_PTR on error; update the documentation to match.

The documentation comment for ext4_wait_block_bitmap describes it as
returning 1 on error, but it returns -errno on error; update the
documentation to match.

Signed-off-by: Josh Triplett <[email protected]>
Reviewed-by: Ritesh Harani <[email protected]>
Link: https://lore.kernel.org/r/60a3f4996f4932c45515aaa6b75ca42f2a78ec9b.1585512514.git.josh@joshtriplett.org
Signed-off-by: Theodore Ts'o <[email protected]>