quota: use flags interface for dquot alloc/free space

Switch __dquot_alloc_space and __dquot_free_space to take flags
to indicate whether to warn and/or to reserve (or free reserve).

This is slightly more readable at the callpoints, and makes it
cleaner to add a "nofail" option in the next patch.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>