ext4: only set project inherit bit for directory

It doesn't make any sense to have project inherit bits
for regular files, even though this won't cause any
problem, but it is better fix this.

Signed-off-by: Wang Shilong <wshilong@ddn.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>