ext4: fix error handling in ext4_ind_get_blocks()

When an error happened in ext4_splice_branch we failed to notice that
in ext4_ind_get_blocks and mapped the buffer anyway. Fix the problem
by checking for error properly.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: stable@kernel.org