debugfs: fix various minor bogosity

debugfs: fix various minor bogosity

We should really use the ext2fs memory allocator functions in
copy_file(), and we really should return a value if there's allocation
problems.

Also fix up a minor bogosity in an error message.

Reviewed-by: Zheng Liu <wenqing.lz@taobao.com>
Cc: Robert Yang <liezhi.yang@windriver.com>
Cc: Darren Hart <dvhart@linux.intel.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>