[PATCH] add discard option -d to swapon

[Matt Whitlock]

Attached are a series of three patches affecting swaponoff.

Patch #1 fixes two small logic errors: (a) the bb_strtou function was called inside an instantiation of the MIN macro, which would cause the function to be called twice in the common case; and (b) the actual maximum allowable swap priority is SWAP_FLAG_PRIO_MASK >> SWAP_FLAG_PRIO_SHIFT, not simply SWAP_FLAG_PRIO_MASK as the code previously assumed. It just so happens that SWAP_FLAG_PRIO_SHIFT == 0, but one should not rely on that serendipity.

Patch #2 fixes the interaction of the -a and -p options to swapon. When -p is given on the command line, the specified priority should be applied to any swap areas given in /etc/fstab that lack a 'pri' option. This is the way swapon from util-linux does it, and it's the rational behavior, or else -p and -a should be mutually exclusive options.

Patch #3 adds a -d option to swapon, which sets SWAP_FLAG_DISCARD and potentially SWAP_FLAG_DISCARD_ONCE or SWAP_FLAG_DISCARD_PAGES if a policy is given. The patch also adds support for the 'discard' option in swap entries in /etc/fstab.
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