overlayfs: Use userxattrs on supporting kernels.

In previous kernels, overlayfs stored its xattrs with a "trusted."
prefix. This requires CAP_SYS_ADMIN. As a workaround, we carried
out-of-tree kernel patches to bypass the security checks on these attrs.

The 5.15 kernel however has a new mount option "userxattr". When this is
set, the "trusted." prefix is replaced with "user.", which eliminates
the CAP_SYS_ADMIN requirement.

On kernels >= 5.15 we can use this feature and drop some of our
out-of-tree patches.

Bug: 204981027
Test: adb remount on cuttlefish with >=5.15
Change-Id: I3f0ca637a62c949fe481eea84f2c682f1ff4517a