LU-2634 ldiskfs: clear extent flag for fast symlink

Fast symlinks on MDT filesystems formatted with the "extents" feature
are created with EXT4_EXTENTS_FL. e2fsck considers this a corruption
and deletes the symlink.

The patch is to clear LDISKFS_INODE_EXTENTS flag in
osd_ldiskfs_writelink() just as ext4_symlink() does.

Signed-off-by: Liu Ying <emoly.liu@intel.com>
Change-Id: Ib7c1eee7becfaf2b80805d3f6c13658686ad5699
Reviewed-on: http://review.whamcloud.com/5154
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>