fs: fix new splice.c kernel-doc warning

Fix new kernel-doc warning in fs/splice.c:

  Warning(fs/splice.c:1298): No description found for parameter 'opos'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>