fs: fix namei.c kernel-doc notation

Fix new kernel-doc notation warnings in fs/namei.c and spell
ECHILD correctly.

  Warning(fs/namei.c:218): No description found for parameter 'flags'
  Warning(fs/namei.c:425): Excess function parameter 'Returns' description in 'nameidata_drop_rcu'
  Warning(fs/namei.c:478): Excess function parameter 'Returns' description in 'nameidata_dentry_drop_rcu'
  Warning(fs/namei.c:540): Excess function parameter 'Returns' description in 'nameidata_drop_rcu_last'

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc:	Nick Piggin <npiggin@kernel.dk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>