Fix vfs_busy leaks in ufs_quotactl

[cse_cem_gmail_com]

Summary

The sys_quotactl contract demands that the mp is vfs_unbusy()ed when the
cmd is Q_QUOTAON, regardless of other input parameters or error return.

Sponsored by: EMC / Isilon Storage Division

Test Plan

pho's quota9.sh test from stress2 easily and quickly reproduces this issue and confirms the fix:

http://svn.freebsd.org/base/user/pho/stress2/misc/quota9.sh

[kib]

Yes, the fix looks correct.

[pho]

Problem verified http://people.freebsd.org/~pho/stress/log/quota9.txt.
Patch tested OK on GENERIC and non QUOTA build.

[kib]

r277794

[cse_cem_gmail_com]

Thanks!