xfs: fix iget/irele usage in online fsck [v24.5]

This patchset fixes a handful of problems relating to how we get and
release incore inodes in the online scrub code.  The first patch fixes
how we handle DONTCACHE -- our reasons for setting (or clearing it)
depend entirely on the runtime environment at irele time.  Hence we can
refactor iget and irele to use our own wrappers that set that context
appropriately.

The second patch fixes a race between the iget call in the inode core
scrubber and other writer threads that are allocating or freeing inodes
in the same AG by changing the behavior of xchk_iget (and the inode core
scrub setup function) to return either an incore inode or the AGI buffer
so that we can be sure that the inode cannot disappear on us.

The final patch elides MMAPLOCK from scrub paths when possible.  It did
not fit anywhere else.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
-----BEGIN PGP SIGNATURE-----

iHUEABYKAB0WIQQ2qTKExjcn+O1o2YRKO3ySh0YRpgUCZDdPcAAKCRBKO3ySh0YR
piKpAP43yHHymFsXykAj23/YTBumdWYvAYu/n3IDLHe8Tt/UTgEAuiJj/MWfi3aR
35JL0XbhANm9i72vfwRJJl0WkCwwwAI=
=zQ5R
-----END PGP SIGNATURE-----