fscache: fix misdocumented parameter

This patch fixes two warnings generated by make docs. The functions
fscache_use_cookie and fscache_unuse_cookie, both have a parameter
named cookie. But they are documented with the name "object" with
unclear description. Which generates the warning when creating docs.

This commit will replace the currently misdocumented parameter names
with the correct ones while adding proper descriptions.

CC: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Khalid Masum <khalid.masum.92@gmail.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/r/20220521142446.4746-1-khalid.masum.92@gmail.com/ # v1
Link: https://lore.kernel.org/r/20220818040738.12036-1-khalid.masum.92@gmail.com/ # v2
Link: https://lore.kernel.org/r/880d7d25753fb326ee17ac08005952112fcf9bdb.1657360984.git.mchehab@kernel.org/ # Mauro's version