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 <[e-mail]>
Signed-off-by: Viacheslav Dubeyko <[e-mail]>

include/uapi/linux/nilfs2_api.h

···
 NILFS_CPINFO_FNS(MINOR, minor)
 
 /**
- * nilfs_suinfo - segment usage information
+ * struct nilfs_suinfo - segment usage information
  * @sui_lastmod: timestamp of last modification
  * @sui_nblocks: number of written blocks in segment
  * @sui_flags: segment usage flags
···
 }
 
 /**
- * nilfs_suinfo_update - segment usage information update
+ * struct nilfs_suinfo_update - segment usage information update
  * @sup_segnum: segment number
  * @sup_flags: flags for which fields are active in sup_sui
  * @sup_reserved: reserved necessary for alignment