fs: FS_POSIX_ACL does not depend on BLOCK

- Fix a kconfig unmet dependency warning.
- Remove the comment that identifies which filesystems use POSIX ACL
  utility routines.
- Move the FS_POSIX_ACL symbol outside of the BLOCK symbol if/endif block
  because its functions do not depend on BLOCK and some of the filesystems
  that use it do not depend on BLOCK.

warning: (GENERIC_ACL && JFFS2_FS_POSIX_ACL && NFSD_V4 && NFS_ACL_SUPPORT && 9P_FS_POSIX_ACL) selects FS_POSIX_ACL which has unmet direct dependencies (BLOCK)

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>