f2fs crypto: avoid f2fs_inherit_context for symlink

This patch fixes to call f2fs_inherit_context twice for newly created symlink.
The original one is called by f2fs_add_link(), which invokes f2fs_setxattr.
If the second one is called again, f2fs_setxattr is triggered again with same
encryption index.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>