nilfs2: fix missing struct keywords in nilfs2_api.h kernel-doc

nilfs2: fix missing struct keywords in nilfs2_api.h kernel-doc

Eliminate the following kernel-doc warnings in nilfs2_api.h:

Warning: include/uapi/linux/nilfs2_api.h:65 cannot understand function
 prototype: 'struct nilfs_suinfo'
Warning: include/uapi/linux/nilfs2_api.h:101 cannot understand function
 prototype: 'struct nilfs_suinfo_update'

This ensures that the documentation for nilfs_suinfo and
nilfs_suinfo_update is correctly parsed and generated by adding the
missing 'struct' keyword to their kernel-doc comments.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Signed-off-by: Viacheslav Dubeyko <slava@dubeyko.com>