LU-16061 osd-ldiskfs: clear EXTENT_FL for symlink agent inode

LU-16061 osd-ldiskfs: clear EXTENT_FL for symlink agent inode

The flag should be cleared for "fast" symlinks otherwise
e2fsck complains about inode correctness.
New agent inodes of symlink type may have EXT4_EXTENT_FL flag
set if the fs has "extent" feature and it is not cleared as in
other places where "fast" symlinks are created.

HPE-bug-id: LUS-10237

Signed-off-by: Alexander Zarochentsev <alexander.zarochentsev@hpe.com>
Change-Id: Ib7b807bb1298cc3a9fd4fdba35747b4bda6fe034
Reviewed-on: https://review.whamcloud.com/48093
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Artem Blagodarenko <ablagodarenko@ddn.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>