p9: avoid unused variable warning

Commit 4e34e719 ("fs: take the ACL checks to common code") removed
the use of the 'acl' variable in v9fs_iop_get_acl(), but left the
variable definition around.  Remove it to get rid of the warning:

  fs/9p/acl.c: In function ‘v9fs_iop_get_acl’:
  fs/9p/acl.c:101:20: warning: unused variable ‘acl’

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>