quota: Fix warning when a delayed write happens before quota is enabled

If a delayed-allocation write happens before quota is enabled, the
kernel spits out a warning:
WARNING: at fs/quota/dquot.c:988 dquot_claim_space+0x77/0x112()

because the fact that user has some delayed allocation is not recorded
in quota structure.

Make dquot_initialize() update amount of reserved space for user if it sees
inode has some space reserved. Also make sure that reserved quota space does
not go negative and we warn about the filesystem bug just once.

Signed-off-by: Jan Kara <jack@suse.cz>